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/hugo_sites.go

@@ -57,6 +57,14 @@ func (h *HugoSites) IsMultihost() bool {
 	return h != nil && h.multihost
 }
 
+func (h *HugoSites) LanguageSet() map[string]bool {
+	set := make(map[string]bool)
+	for _, s := range h.Sites {
+		set[s.Language.Lang] = true
+	}
+	return set
+}
+
 func (h *HugoSites) NumLogErrors() int {
 	if h == nil {
 		return 0