Add directory based archetypes

Given this content:

```bash
archetypes
├── default.md
└── post-bundle
    ├── bio.md
    ├── images
    │   └── featured.jpg
    └── index.md
```

```bash
hugo new --kind post-bundle post/my-post
```

Will create a new folder in `/content/post/my-post` with the same set of files as in the `post-bundle` archetypes folder.

This commit also improves the archetype language detection, so, if you use template code in your content files, the `.Site` you get is for the correct language. This also means that it is now possible to translate strings defined in  the `i18n` bundles,  e.g. `{{ i18n "hello" }}`.

Fixes #4535

hugolib/fileInfo.go

@@ -61,7 +61,7 @@ func (fi *fileInfo) isOwner() bool {
 	return fi.bundleTp > bundleNot
 }
 
-func isContentFile(filename string) bool {
+func IsContentFile(filename string) bool {
 	return contentFileExtensionsSet[strings.TrimPrefix(helpers.Ext(filename), ".")]
 }
 
@@ -98,7 +98,7 @@ const (
 // Returns the given file's name's bundle type and whether it is a content
 // file or not.
 func classifyBundledFile(name string) (bundleDirType, bool) {
-	if !isContentFile(name) {
+	if !IsContentFile(name) {
 		return bundleNot, false
 	}
 	if strings.HasPrefix(name, "_index.") {