Support per-document locale permalinks and config

[jaredcwhite]

Partially addresses #49

This PR does four important things:

1. It sets the locale front matter of a document (if it's not set explicitly) by looking for language or lang front matter or finally the filename pattern slug.locale.ext (which might be the preferable way to do things).
2. It strips out the last locale slug segment (if present) when generating the destination URL/path.
3. You can configure posts or other collection permalinks to output to a dedicated top-level folder when a document locale is set…e.g., _collection/my-cool-doc.es.md would become /es/collection/my-cool-doc, whereas _collection/my-cool-doc.md would remain /collection/my-cool-doc
4. When a document locale is set, site.locale is set to the document locale when that document is being rendered, so you can take advantage of the site.locale value in layout templates, components, partials, using the t translation helper, etc.

Note: none of this applies to pages at present, only documents (essentially any content type not a page).

There's more work to do to investigate patterns and potential fixes needed across the whole codebase to handle multilingual content (for example, if you want to loop through blog posts on language-specific index pages using pagination…how do you do that?), but this is a good step in the right direction.