--override-ini now correctly overrides some fundamental options like "python_files"

[nicoddemus]

#2238

[coveralls]

Coverage increased (+0.0008%) to 92.709% when pulling 9eb1d73 on nicoddemus:override-python-files into 3d9c5cf on pytest-dev:master.

[eli-b]

+1

This also enables overriding addopts.
I have plugin-specific options in my addopts. If I use pytest -p no:my_plugin, pytest fails because of the now-unknown options in my addopts. This commit would allow me to turn off plugins without having to temporarily delete my pytest.ini.