Allow disabling active config files.

[brandtbucher]

With this change, passing --config-file= from the command line disables any active config files.

[gvanrossum]

Why not use /dev/null?

[brandtbucher]

$ mypy --config-file /dev/null .
/dev/null: No [mypy] section in config file

This cuts down on unnecessary noise, and to me it personally feels like less of a hack.

[emmatyping]

I think this makes sense. In addition, you won't have to remember NUL vs /dev/null. Maybe we should document this behavior if we merge this?

[gvanrossum]

OK, this sounds like a good idea. But yeah, please update the docs. Also please add a unittest to test-data/unit/check-flags.test, e.g. show that options in mypy.ini are ignored with --config-file=.

[brandtbucher]

Ok, added documentation and testing!

[gvanrossum]

Thanks! All looks good.