[WIP] Add support for sourcemaps from autoprefixer.

Hey @sindresorhus - I started doing this and got stuck, and figured I'd send it your way. I'm pretty new to broccoli.

This totally works, except only for one file. Because the options can contain the sourcemap of the first preprocessor, you can only pass the previous map of one file. Is there a way in Broccoli to change this configuration option per file that is processed?

Let me know if that doesn't make sense.