Configure Sass to load from theme-gem if possible

[ashmaroli]

Add _sass from within theme-gems to Sass.load_paths if the current site uses a theme gem and the theme-gem contains _sass directory at its root.

[DirtyF]

@ashmaroli Can you please resolve the conflicts, I'd like us to ship a new release of this gem with sassc support.

[ashmaroli]

@DirtyF I intentionally let this slide because I'm not sure if this is the right approach.. jekyll:master is still hard-coded to use the EOL Sass library when using theme-gems in a Jekyll build..

That said, as of now, the master branch of this project is not fully compatible with Jekyll builds on Windows (based on the latest AppVeyor results).

If you must release a new version of this plugin despite the above, go ahead. This PR can be shipped later..

[DirtyF]

@jekyllbot: merge +minor