Add --syntax=sass option to Install task

I submit this update for review. It adds a `--syntax=[syntax]` option to `bitters install` which defaults to `scss` but allows a user to specify the `sass` syntax if they wish. It works by globbing for `scss` files in the newly created directory, running Sass's `sass-convert` command and deleting the leftover `scss` files. Sass comes with `sass-convert` so I felt okay about relying on its presence. Although if someone were using only `libsass` maybe that command wouldn't be available.

I look forward to feedback on the idea.