Added support for sass imports underscore naming convention.

I had some issues getting Sass working with Brunch, and eventually I figured out that changing

    @import "foo/bar";

to

    @import "foo/_bar";

 fixed the issues.

But having `@import "myfile";` resolve to `_myfile.scss` is actually a Sass naming convention (http://sass-lang.com/tutorial.html#id1), so I patched sass-brunch to look for extra dependencies that might exist.