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).


@RaymondLim It looks like we'll run all thise code on every document change, even when Live Preview is not active (or has never even been invoked before).  I'm guessing it won't cause any bugs -- because `_server` is null, `_styleSheetAdded()` will get a null `path` & bail early -- but it seems cleaner & faster to just have the listener attached only when Live Preview is active.  Is that a cleanup that could be rolled into #8844 too?