How to make brushExtent() work with ordinal data?

Hey @BigFatDog, Thanks for developing this awesome library. 

I am wondering if you could implement the change in `brushExtent()` to load data from ordinal scales. Currently, it fails because to my understanding you expect scaled and inverted values for input. 

It may be a good idea to get raw selection inputs and make it work on ordinal scale as well. 

