Sourcemap support

[onelson]

Hi there!

I actually started using https://github.com/ShaggyYeti/sbt-sass a couple days ago and was able to add sourcemap support (see my pull request: ShaggyYeti/sbt-sass#10).

Once I realized this other project was more-or-less a clone copy of this one, I thought I'd give it a shot here too! Unfortunately I haven't been able to quite grok how this should work with the way you have things set up here, so I thought I'd just file an issue instead.

Mainly, it seems like the play.AssetsCompiler trait, which I've been unable to find docs/source for, is doing something with the generated files to either move them or write them or I'm not sure what's going on here. For sourcemaps in sass, you need to explicitly specify the infile path and the outfile path.

Anyway, I'm happy to attempt this again if I can get some guidance. Otherwise, perhaps my PR to sbt-sass will be enough for you to work from.