Rework external asciidoctor integration

This commit solves the relative path problem with asciidoctor tooling. An include will resolve relatively, so you can refer easily to files in the same folder.

Also `asciidoctor-diagram` and PlantUML rendering works now, because the created temporary files will be placed in the correct folder.

This patch covers just the Ruby version of asciidoctor. The old AsciiDoc CLI EOLs in Jan 2020, so this variant is removed from code.

The configuration is completely rewritten and now available in `config.toml` under the key `[markup.asciidocext]`:

```toml
[markup.asciidocext]
    extensions = ["asciidoctor-html5s", "asciidoctor-diagram"]
    workingFolderCurrent = true
    trace = true
    [markup.asciidocext.attributes]
        my-base-url = "https://example.com/"
        my-attribute-name = "my value"
```

- backends, safe-modes, and extensions are now whitelisted to the popular (ruby) extensions and valid values.
- the default for extensions is to not enable any, because they're all external dependencies so the build would break if the user didn't install them beforehand.
- the default backend is html5 because html5s is an external gem dependency.
- the default safe-mode is safe, explanations of the modes: https://asciidoctor.org/man/asciidoctor/
- the config is namespaced under asciidocext_config and the parser looks at asciidocext to allow a future native Go asciidoc.
- `uglyUrls=true` option and `--source` flag are supported
- `--destination` flag is required

Follow the updated documentation under `docs/content/en/content-management/formats.md`. 
  
This patch would be a breaking change, because you need to correct all your absolute include pathes to relative paths, so using relative paths must be configured explicitly by setting `workingFolderCurrent = true`.

1 files changed, 83 insertions, 0 deletions

diff --git a/markup/asciidocext/asciidocext_config/config.go b/markup/asciidocext/asciidocext_config/config.go
new file mode 100644
index 000000000..8cc3e79e6
--- /dev/null
+++ b/markup/asciidocext/asciidocext_config/config.go
@@ -0,0 +1,83 @@
+// Copyright 2020 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 asciidoc_config holds asciidoc related configuration.
+package asciidocext_config
+
+// DefaultConfig holds the default asciidoc configuration.
+// These values are asciidoctor cli defaults (see https://asciidoctor.org/docs/user-manual/)
+var (
+	Default = Config{
+		Backend:              "html5",
+		DocType:              "article",
+		Extensions:           []string{},
+		Attributes:           map[string]string{},
+		NoHeaderOrFooter:     true,
+		SafeMode:             "unsafe",
+		SectionNumbers:       false,
+		Verbose:              true,
+		Trace:                false,
+		FailureLevel:         "fatal",
+		WorkingFolderCurrent: false,
+	}
+
+	AllowedExtensions = map[string]bool{
+		"asciidoctor-html5s":           true,
+		"asciidoctor-diagram":          true,
+		"asciidoctor-interdoc-reftext": true,
+		"asciidoctor-katex":            true,
+		"asciidoctor-latex":            true,
+		"asciidoctor-question":         true,
+		"asciidoctor-rouge":            true,
+	}
+
+	AllowedSafeMode = map[string]bool{
+		"unsafe": true,
+		"safe":   true,
+		"server": true,
+		"secure": true,
+	}
+
+	AllowedFailureLevel = map[string]bool{
+		"fatal": true,
+		"warn":  true,
+	}
+
+	AllowedBackend = map[string]bool{
+		"html5":     true,
+		"html5s":    true,
+		"xhtml5":    true,
+		"docbook5":  true,
+		"docbook45": true,
+		"manpage":   true,
+	}
+
+	DisallowedAttributes = map[string]bool{
+		"outdir": true,
+	}
+)
+
+// Config configures asciidoc.
+type Config struct {
+	Backend              string
+	DocType              string
+	Extensions           []string
+	Attributes           map[string]string
+	NoHeaderOrFooter     bool
+	SafeMode             string
+	SectionNumbers       bool
+	Verbose              bool
+	Trace                bool
+	FailureLevel         string
+	WorkingFolderCurrent bool
+}