Add the ability to override the mentionable tag with a custom template

Add translation
Fix bug where a setting will not save
This close #4

So I went with an approche of using a template instead of actually putting the template markup in the option table of the database. This allow for a greater flexibility as the themer will access to the post object.

@westonruter Would you mind reviewing this and giving me feedback?
/cc @shadyvb @topher1kenobe