[Feature Request] Support custom pypi registries

[robross0606]

Is your feature request related to a problem/bug/intentional feature? Please describe if it is.

In many environments, users go to a proxy instead of directly to pypi.org. They also may be supplementing internal packages in their private registries. pip supports this by way of .pypirc, pip.conf/pip.ini and $PIP_INDEX_URL, but this project seems to ignore those settings and always uses pypi.org.

Describe the solution you'd like.

Ideally, the project would pay attention to the same configuration as pip and check the expected repositories instead of always using pypi.org.

Describe alternatives you've considered if there are any.

A direct command-line switch to override the repository URL would be marginally useful in lieu of directly using pip config files.

Additional context if applicable

You can see what pip checks by calling pip config list -v:

# pip config list -v
For variant 'global', will try loading '/etc/xdg/pip/pip.conf'
For variant 'global', will try loading '/etc/pip.conf'
For variant 'user', will try loading '/home/user/.pip/pip.conf'
For variant 'user', will try loading '/home/user/.config/pip/pip.conf'
For variant 'site', will try loading '/usr/pip.conf'