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).


We should clarify in this comment that this behavior is special for preprocessors - i.e., for normal CSS we only do highlighting from files we know for sure are referenced by the current live preview document, but for preprocessors we just assume that any preprocessor file you edit is probably related to the live preview.