Support for pip.conf

Would be neat if honesty respected the configured pypi mirror, which is typically in /etc/pip.conf or ~/.config/pip/pip.conf (and more places if you want to be exhaustive... but these are the 2 main ones).

An alternative way would be to get it via pip config get global.index-url

Looks like the locations are fairly well-defined, this is almost worth making a tiny project whose job it is to check all of them in the right order to find index_url.