Adding ability to specify render options.

**NOTE**: Credit is due to @ejholmes as he got to this first in #18; I've just cleaned it up a bit. Happy to rebase if you want to merge his PR (just ping me).

This pull request adds the ability to specify render options in your `Gruntfile`.

I understand you're waiting to merge until @andrew publishes a new node-sass (as, until then, this build will break). Figured I'd queue this up for when he does.