Added "Varied Fill" brush. Added "Fill Air" checkbox to both Fill and varied Varied Fill brush.

Done :-) Tested it quite a few times, seems to work smooth.

Btw, is there any way to upload my edited code or something? Right now I have to go to my fork, then click "edit" on my forked brush.py, and CTRL+C CTRL+V my own code, and then add a pull request. Must be a better way to do that?