How to make brushExtent() work with ordinal data?

[divyaeM]

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.