Add support for theme composition and inheritance

This commit adds support for theme composition and inheritance in Hugo.

With this, it helps thinking about a theme as a set of ordered components:

```toml
theme = ["my-shortcodes", "base-theme", "hyde"]
```

The theme definition example above in `config.toml` creates a theme with the 3 components with presedence from left to right.

So, Hugo will, for any given file, data entry etc., look first in the project, and then in `my-shortcode`, `base-theme` and lastly `hyde`.

Hugo uses two different algorithms to merge the filesystems, depending on the file type:

* For `i18n` and `data` files, Hugo merges deeply using the translation id and data key inside the files.
* For `static`, `layouts` (templates) and `archetypes` files, these are merged on file level. So the left-most file will be chosen.

The name used in the `theme` definition above must match a folder in `/your-site/themes`, e.g. `/your-site/themes/my-shortcodes`. There are  plans to improve on this and get a URL scheme so this can be resolved automatically.

Also note that a component that is part of a theme can have its own configuration file, e.g. `config.toml`. There are currently some restrictions to what a theme component can configure:

* `params` (global and per language)
* `menu` (global and per language)
* `outputformats` and `mediatypes`

The same rules apply here: The left-most param/menu etc. with the same ID will win. There are some hidden and experimental namespace support in the above, which we will work to improve in the future, but theme authors are encouraged to create their own namespaces to avoid naming conflicts.

A final note: Themes/components can also have a `theme` definition in their `config.toml` and similar, which is the "inheritance" part of this commit's title. This is currently not supported by the Hugo theme site. We will have to wait for some "auto dependency" feature to be implemented for that to happen, but this can be a powerful feature if you want to create your own theme-variant based on others.

Fixes #4460
Fixes #4450

2 files changed, 265 insertions, 0 deletions

diff --git a/langs/language.go b/langs/language.go
new file mode 100644
index 000000000..6f3e1de64
--- /dev/null
+++ b/langs/language.go
@@ -0,0 +1,217 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package langs
+
+import (
+	"sort"
+	"strings"
+
+	"github.com/gohugoio/hugo/common/maps"
+	"github.com/gohugoio/hugo/config"
+	"github.com/spf13/cast"
+)
+
+// These are the settings that should only be looked up in the global Viper
+// config and not per language.
+// This list may not be complete, but contains only settings that we know
+// will be looked up in both.
+// This isn't perfect, but it is ultimately the user who shoots him/herself in
+// the foot.
+// See the pathSpec.
+var globalOnlySettings = map[string]bool{
+	strings.ToLower("defaultContentLanguageInSubdir"): true,
+	strings.ToLower("defaultContentLanguage"):         true,
+	strings.ToLower("multilingual"):                   true,
+}
+
+// Language manages specific-language configuration.
+type Language struct {
+	Lang         string
+	LanguageName string
+	Title        string
+	Weight       int
+
+	Disabled bool
+
+	// If set per language, this tells Hugo that all content files without any
+	// language indicator (e.g. my-page.en.md) is in this language.
+	// This is usually a path relative to the working dir, but it can be an
+	// absolute directory referenece. It is what we get.
+	ContentDir string
+
+	Cfg config.Provider
+
+	// These are params declared in the [params] section of the language merged with the
+	// site's params, the most specific (language) wins on duplicate keys.
+	params map[string]interface{}
+
+	// These are config values, i.e. the settings declared outside of the [params] section of the language.
+	// This is the map Hugo looks in when looking for configuration values (baseURL etc.).
+	// Values in this map can also be fetched from the params map above.
+	settings map[string]interface{}
+}
+
+func (l *Language) String() string {
+	return l.Lang
+}
+
+// NewLanguage creates a new language.
+func NewLanguage(lang string, cfg config.Provider) *Language {
+	// Note that language specific params will be overridden later.
+	// We should improve that, but we need to make a copy:
+	params := make(map[string]interface{})
+	for k, v := range cfg.GetStringMap("params") {
+		params[k] = v
+	}
+	maps.ToLower(params)
+
+	defaultContentDir := cfg.GetString("contentDir")
+	if defaultContentDir == "" {
+		panic("contentDir not set")
+	}
+
+	l := &Language{Lang: lang, ContentDir: defaultContentDir, Cfg: cfg, params: params, settings: make(map[string]interface{})}
+	return l
+}
+
+// NewDefaultLanguage creates the default language for a config.Provider.
+// If not otherwise specified the default is "en".
+func NewDefaultLanguage(cfg config.Provider) *Language {
+	defaultLang := cfg.GetString("defaultContentLanguage")
+
+	if defaultLang == "" {
+		defaultLang = "en"
+	}
+
+	return NewLanguage(defaultLang, cfg)
+}
+
+// Languages is a sortable list of languages.
+type Languages []*Language
+
+// NewLanguages creates a sorted list of languages.
+// NOTE: function is currently unused.
+func NewLanguages(l ...*Language) Languages {
+	languages := make(Languages, len(l))
+	for i := 0; i < len(l); i++ {
+		languages[i] = l[i]
+	}
+	sort.Sort(languages)
+	return languages
+}
+
+func (l Languages) Len() int           { return len(l) }
+func (l Languages) Less(i, j int) bool { return l[i].Weight < l[j].Weight }
+func (l Languages) Swap(i, j int)      { l[i], l[j] = l[j], l[i] }
+
+// Params retunrs language-specific params merged with the global params.
+func (l *Language) Params() map[string]interface{} {
+	return l.params
+}
+
+// IsMultihost returns whether there are more than one language and at least one of
+// the languages has baseURL specificed on the language level.
+func (l Languages) IsMultihost() bool {
+	if len(l) <= 1 {
+		return false
+	}
+
+	for _, lang := range l {
+		if lang.GetLocal("baseURL") != nil {
+			return true
+		}
+	}
+	return false
+}
+
+// SetParam sets a param with the given key and value.
+// SetParam is case-insensitive.
+func (l *Language) SetParam(k string, v interface{}) {
+	l.params[strings.ToLower(k)] = v
+}
+
+// GetBool returns the value associated with the key as a boolean.
+func (l *Language) GetBool(key string) bool { return cast.ToBool(l.Get(key)) }
+
+// GetString returns the value associated with the key as a string.
+func (l *Language) GetString(key string) string { return cast.ToString(l.Get(key)) }
+
+// GetInt returns the value associated with the key as an int.
+func (l *Language) GetInt(key string) int { return cast.ToInt(l.Get(key)) }
+
+// GetStringMap returns the value associated with the key as a map of interfaces.
+func (l *Language) GetStringMap(key string) map[string]interface{} {
+	return cast.ToStringMap(l.Get(key))
+}
+
+// GetStringMapString returns the value associated with the key as a map of strings.
+func (l *Language) GetStringMapString(key string) map[string]string {
+	return cast.ToStringMapString(l.Get(key))
+}
+
+//  returns the value associated with the key as a slice of strings.
+func (l *Language) GetStringSlice(key string) []string {
+	return cast.ToStringSlice(l.Get(key))
+}
+
+// Get returns a value associated with the key relying on specified language.
+// Get is case-insensitive for a key.
+//
+// Get returns an interface. For a specific value use one of the Get____ methods.
+func (l *Language) Get(key string) interface{} {
+	local := l.GetLocal(key)
+	if local != nil {
+		return local
+	}
+	return l.Cfg.Get(key)
+}
+
+// GetLocal gets a configuration value set on language level. It will
+// not fall back to any global value.
+// It will return nil if a value with the given key cannot be found.
+func (l *Language) GetLocal(key string) interface{} {
+	if l == nil {
+		panic("language not set")
+	}
+	key = strings.ToLower(key)
+	if !globalOnlySettings[key] {
+		if v, ok := l.settings[key]; ok {
+			return v
+		}
+	}
+	return nil
+}
+
+// Set sets the value for the key in the language's params.
+func (l *Language) Set(key string, value interface{}) {
+	if l == nil {
+		panic("language not set")
+	}
+	key = strings.ToLower(key)
+	l.settings[key] = value
+}
+
+// IsSet checks whether the key is set in the language or the related config store.
+func (l *Language) IsSet(key string) bool {
+	key = strings.ToLower(key)
+
+	key = strings.ToLower(key)
+	if !globalOnlySettings[key] {
+		if _, ok := l.settings[key]; ok {
+			return true
+		}
+	}
+	return l.Cfg.IsSet(key)
+
+}
diff --git a/langs/language_test.go b/langs/language_test.go
new file mode 100644
index 000000000..8783172fb
--- /dev/null
+++ b/langs/language_test.go
@@ -0,0 +1,48 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package langs
+
+import (
+	"testing"
+
+	"github.com/spf13/viper"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetGlobalOnlySetting(t *testing.T) {
+	v := viper.New()
+	v.Set("defaultContentLanguageInSubdir", true)
+	v.Set("contentDir", "content")
+	v.Set("paginatePath", "page")
+	lang := NewDefaultLanguage(v)
+	lang.Set("defaultContentLanguageInSubdir", false)
+	lang.Set("paginatePath", "side")
+
+	require.True(t, lang.GetBool("defaultContentLanguageInSubdir"))
+	require.Equal(t, "side", lang.GetString("paginatePath"))
+}
+
+func TestLanguageParams(t *testing.T) {
+	assert := require.New(t)
+
+	v := viper.New()
+	v.Set("p1", "p1cfg")
+	v.Set("contentDir", "content")
+
+	lang := NewDefaultLanguage(v)
+	lang.SetParam("p1", "p1p")
+
+	assert.Equal("p1p", lang.Params()["p1"])
+	assert.Equal("p1cfg", lang.Get("p1"))
+}