Add Coffee Extras submodule

As previously discussed in issue #2362475, I maintain a small module called Coffee Extras (currently a sandbox) which provides some extra shortcuts for managing bundle fields, taxonomies, and more.

To avoid module duplication, I think the best approach for releasing this functionality would not be as a separate module, but as an optional submodule inside of Coffee. So I'm attaching a patch to provide the submodule. Let me know what you think. Thanks!