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/site.go

@@ -795,7 +795,7 @@ func (s *Site) processPartial(events []fsnotify.Event) (whatChanged, error) {
 				removed = true
 			}
 		}
-		if removed && isContentFile(ev.Name) {
+		if removed && IsContentFile(ev.Name) {
 			h.removePageByFilename(ev.Name)
 		}