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) with the exception of the following unchecked remaining development tasks.

- [ ] Don't show Quick Edit results for both generated CSS _and_ original SCSS/LESS file
- [x] Hook up nested selector support to Live Preview Highlight
- [ ] Unit tests: CSSUtils APIs with LESS/SCSS
- [ ] Unit tests: Quick Edit with LESS/SCSS
- [ ] Unit tests: Live Preview Highlight with LESS/SCSS ?


There are many different kinds of "preprocessors" -- maybe changing this to `CSSPreprocessorDocument` would be more descriptive and allow for other kinds of non-css preprocessors (e.g. Jade).