Add translation options for posts

Optional translation interface for posts. To use, add language options in `_config.yml`:

```yml
languages:
  - name: 'English'
    key: 'en'
  - name: 'Español'
    key: 'es'
  - name: 'Français'
    key: 'fr'
```

Set each post's language with in its frontmatter, like `lang: en`, and save posts with the same file name into a subdirectory based on their language code:

```
/_posts/2013-01-01-post-title.md
/_posts/es/2013-01-01-post-title.md
/_posts/fr/2013-01-01-post-title.md
```