feat: allow setting 'files' argument in the config file

[red-hood]

As mentioned in #239, theses changes allow passing the 'files' argument in the config file, calling autoflake without any arguments. The config file is then searched in the current working directory.

This might be useful e.g. in a CI context, where autoflake is called from a shared template with fixed arguments, but source folder layout varies between different repositories.

[red-hood]

Thank you for running the tests. I decided to not continue with this effort, as usind plain unittest without the monkeypatch feature of pytest makes changing the cwd rather complicated and error prone in parallel tests.

[fsouza]

Thank you for running the tests. I decided to not continue with this effort, as usind plain unittest without the monkeypatch feature of pytest makes changing the cwd rather complicated and error prone in parallel tests.

I'd be OK with moving away from unittest, but I understand that that's also a major undertaking.