Add the ability for categorization of react-select options

[reynolek]

I wrote this mostly for our use at work. I personally find this code useful and figured I would take the time to deal with testing it and adding a couple examples to the dev server.

All of the tests are essentially copies of previous tests, with the categorization flag added. I used this as my approach to make sure adding categorization did not break anything else. The one exception is allowing create with categorization -- The code logic wouldn't (easily) be able to identify which category to place the created option in.

Let me know if you have any questions.

[JedWatson]

Sorry this never got attended to. With the release of 1.0.0 I'm closing a lot of old PRs and Issues that either may be fixed / or won't be merged given the major changes since.

Please re-open if you still want to address this in the latest version.

Thanks!