Multi-file content document localization

[jeremydw]

In addition to split documents (by --- and $locale) and @ notation for yaml-formatted documents from #142 , it should be possible to localize content files by duplicating the file and annotating the filename with @<locale>.

For example:

page.md
page@fr.md  # French version of `page.md`.

Grow will treat these localized files as if they are no different from currently-implemented "split-document" localized files. In the above example, page@fr.md implies a configuration of $locale: fr, but otherwise it will work exactly the same as Grow's current split-document localization works (http://grow.io/docs/content-localization/#markdown-body-localization) and no other changes would occur.

The intent of this feature is to make it easier to manage the localization of very large Markdown documents, though it can also be used to split localizations of yaml-formatted documents into multiple files if the user desires.

This should function somewhat similarly to octopress/multilingual (https://github.com/octopress/multilingual#site-template-language-dictionaries) or, less similarly, to Jekyll plugins that allow you to store manage Markdown files on a per-language basis in different folders (https://github.com/Anthony-Gaudino/jekyll-multiple-languages-plugin#42-folder-structure). Grow's implementation would not span folders but rather would simply let the user decorate filenames within the same folder.

[jeremydw]

This was released in https://github.com/grow/grow/releases/tag/0.0.61