Add less/scss/sass support

[mreq]

Shouldn't be much of a problem. Might do it myself once I get some time.

[subhaze]

@mreq I had debated doing that in another package to keep preprocessor separate from vanilla CSS... But I guess it makes sense to add here. I already have some basic LESS completions working in a personal package that I can migrate over.

I'll see about knocking some initial support in this weekend unless you get to it first.