pipreqs circumvents pip.ini; feature request: use pip.ini if it exists.

I'm behind a corporate proxy and use pip.ini to set the proxy. If i run pipreqs without the `--proxy ...` argument, it doesn't find the pypi repos. It would be very cool if pipreqs could use the proxy settings from pip.ini.

Start by locating how pipreqs handles the --proxy argument and how it discovers PyPI repositories. Check how pip.ini settings could be read, then verify that an existing proxy configuration is used when no explicit --proxy argument is provided.

Written by the indexing model from the issue text.