Add Hugo Piper with SCSS support and much more

Before this commit, you would have to use page bundles to do image processing etc. in Hugo.

This commit adds

* A new `/assets` top-level project or theme dir (configurable via `assetDir`)
* A new template func, `resources.Get` which can be used to "get a resource" that can be further processed.

This means that you can now do this in your templates (or shortcodes):

```bash
{{ $sunset := (resources.Get "images/sunset.jpg").Fill "300x200" }}
```

This also adds a new `extended` build tag that enables powerful SCSS/SASS support with source maps. To compile this from source, you will also need a C compiler installed:

```
HUGO_BUILD_TAGS=extended mage install
```

Note that you can use output of the SCSS processing later in a non-SCSSS-enabled Hugo.

The `SCSS` processor is a _Resource transformation step_ and it can be chained with the many others in a pipeline:

```bash
{{ $css := resources.Get "styles.scss" | resources.ToCSS | resources.PostCSS | resources.Minify | resources.Fingerprint }}
<link rel="stylesheet" href="{{ $styles.RelPermalink }}" integrity="{{ $styles.Data.Digest }}" media="screen">
```

The transformation funcs above have aliases, so it can be shortened to:

```bash
{{ $css := resources.Get "styles.scss" | toCSS | postCSS | minify | fingerprint }}
<link rel="stylesheet" href="{{ $styles.RelPermalink }}" integrity="{{ $styles.Data.Digest }}" media="screen">
```

A quick tip would be to avoid the fingerprinting part, and possibly also the not-superfast `postCSS` when you're doing development, as it allows Hugo to be smarter about the rebuilding.

Documentation will follow, but have a look at the demo repo in https://github.com/bep/hugo-sass-test

New functions to create `Resource` objects:

* `resources.Get` (see above)
* `resources.FromString`: Create a Resource from a string.

New `Resource` transformation funcs:

* `resources.ToCSS`: Compile `SCSS` or `SASS` into `CSS`.
* `resources.PostCSS`: Process your CSS with PostCSS. Config file support (project or theme or passed as an option).
* `resources.Minify`: Currently supports `css`, `js`, `json`, `html`, `svg`, `xml`.
* `resources.Fingerprint`: Creates a fingerprinted version of the given Resource with Subresource Integrity..
* `resources.Concat`: Concatenates a list of Resource objects. Think of this as a poor man's bundler.
* `resources.ExecuteAsTemplate`: Parses and executes the given Resource and data context (e.g. .Site) as a Go template.

Fixes #4381
Fixes #4903
Fixes #4858

2 files changed, 4 insertions, 1 deletions

diff --git a/create/content_template_handler.go b/create/content_template_handler.go
index 37eed52cf..02598d4d3 100644
--- a/create/content_template_handler.go
+++ b/create/content_template_handler.go
@@ -134,7 +134,7 @@ func executeArcheTypeAsTemplate(s *hugolib.Site, kind, targetPath, archetypeFile
 		return nil, fmt.Errorf("Failed to parse archetype file %q: %s", archetypeFilename, err)
 	}
 
-	templ := templateHandler.Lookup(templateName)
+	templ, _ := templateHandler.Lookup(templateName)
 
 	var buff bytes.Buffer
 	if err := templ.Execute(&buff, data); err != nil {
diff --git a/create/content_test.go b/create/content_test.go
index e9d46becf..f3bcc1dd5 100644
--- a/create/content_test.go
+++ b/create/content_test.go
@@ -88,6 +88,8 @@ func initViper(v *viper.Viper) {
 	v.Set("i18nDir", "i18n")
 	v.Set("theme", "sample")
 	v.Set("archetypeDir", "archetypes")
+	v.Set("resourceDir", "resources")
+	v.Set("publishDir", "public")
 }
 
 func initFs(fs *hugofs.Fs) error {
@@ -191,6 +193,7 @@ func newTestCfg() (*viper.Viper, *hugofs.Fs) {
 	v.Set("i18nDir", "i18n")
 	v.Set("layoutDir", "layouts")
 	v.Set("archetypeDir", "archetypes")
+	v.Set("assetDir", "assets")
 
 	fs := hugofs.NewMem(v)