Add black as style guide choice

[tonyandrewmeyer]

trafficstars

It would be fantastic if black was available as another style guide choice in the reformat code functionality.

Black is rapidly gaining popularity within the Python world, so seems like an addition that would have wide appeal.

[cclauss]

https://github.com/psf/black has been moved into the Python GitHub domain so it would be nice if Pythonista shipped with it installed. It would be great to have both automatic and manual (a button or menu item) ways to invoke Black on Python files in Pythonista.