Add Included classes in renaming

This PR adds an option to force classes to be included in the renaming map.

The background and motivation for this patch were [discussed on the `closure-stylesheets-discuess` email list](https://groups.google.com/d/msg/closure-stylesheets-discuss/OGum2yekOIc/_Flbc5cO69gJ).

Great to see Closure Stylesheets on GitHub!