Support building multiple targets?

[deliciouslytyped]

It would be nice if autobuilding multiple targets was possible.
I found https://github.com/rowanG077/sphinx-multibuild but didn't test it.

This might make more sense after #151 lands.
At least for me, my main targets are the html and the latexpdf builds.

Then again, this could should also be solvable by just launching sphinx-autobuild multiple times though, and that may be much simpler.