When flake8's config is in setup.cfg, setuptools attempts to set those
options on the command instance. If they don't exist, it fails early
complaining that a specific option does not exist.

This adds this back and does it better than the Flake8 2.x version.

Closes #163

This does not handle setting custom options via the parameters to
get_style_guide.

McCabe reports C90* not C* and Flake8 2 used to exclude .eggs and *.egg

It's redundant and the docs say explicitly that it's ignored.

The intended behaviour already existed.

This reverts commit db9d4ad8b4cc2aab1ef8349dc969faa8f589e9cc.