Fixing Syntax to work with runtime sass file

I've been playing around with Sass recently and got pretty annoyed that some of the finer points of our CSS highlighting didn't work well. The runtime sass file imports this css.vim first, then applies a bunch of stuff on top of it.

This should fix a chunk of those issue, although probably not all (but I will continue to fix this as I find them). The key is that syntax highlighting should be consistent with a normal CSS file, barring a few additions the Sass adds.

Mostly this just involved adding a few `containedin` declarations which should not effect normal CSS in any way.