add support for more fields in setup.cfg

[JensTimmerman]

pip can read all setup options from setup.cfg, and even include files and arguments. This looks like a nice solution to keeping all info in a single place, namely setup.cfg and only call sharedsetup() in the setup.py

see e.g. https://github.com/peterpakos/checkipaconsistency/blob/master/setup.cfg

[JensTimmerman]

this needs a recent setuptools to work, 33.1.1 (the one installed with ez_setup.py) works!