Allow expansion of "~" in paths listed in pip.conf

[rbarrois]

It would be great to be able to set cert = ~/.pip/cacert.pem in .pip/pip.conf, for instance using expanduser.

I'll craft a pull request soon.

[ariddell]

and in commands as well, e.g.: pip wheel --wheel-dir=~/.pip_wheelhouse pyzmq

[dstufft]

Ideally this should be implemented by adding a new optparse type or something similar that will solve it for all paths and not adhoc uses of os.path.expanduser.

[rbarrois]

@uranusjr thanks for fixing this! I really appreciate your help :)