Select Text Localization Override

Add the ability to override select translations without having to copy the entire lang.php file. For example, it may be desirable to change _txt('co') to return "Collaboration" and _txt('cou') to return "Department" without changing most of the other English texts. Ideally, allow this on a per-CO basis.

This could be done via merging lang files or, better yet, via something like CoLocalizationTexts.