This option allows adding extra ignored rules to the default list
instead of replacing it.

The default exclusions are: E121, E123, E126, E226, E24, E704,
W503 and W504.

Refs:
* https://github.com/pypa/setuptools/pull/2486/files#r541943356
* https://flake8.pycqa.org/en/latest/user/options.html#cmdoption-flake8-extend-ignore
* https://flake8.pycqa.org/en/latest/user/options.html#cmdoption-flake8-ignore
* #2501
* https://github.com//skeleton/issues/28