Adds "Include Sass" check box to Advanced generator.

Reference: https://github.com/Automattic/_s/pull/410
Also replace theme headers in Style.scss, not just style.css props @jtsterberg
Get ready for Sass!

Minor changes to @bradp’s PR, to be a little more in sync with the rest
of the code styles.
Merge branch 'master' of https://github.com/Automattic/underscores.me into pr/23
Merge pull request #28 from Automattic/pr/23

underscores.me: Add Sass handling.