Parse less and scss files

This pull request is for [Simple support for LESS/SCSS](https://trello.com/c/KoK6kPtp/1190-m-simple-support-for-less-scss-in-key-brackets-features).


It seems like we should check LanguageManager instead -- don't forget that file extension->language mapping is user configurable and can even be overridden on a per-file basis.