[FEATURE REQUEST] Allow providing mypy config as path to mypy.ini

[okuuva]

mypy tends to get slow with big projects. This can be mitigated by running dmypy which is really useful while developing but doesn't really help in CI where the speed isn't necessarily as important anyway.

It would be nice to be able to use mypy.ini in mypy configuration. This way developer could run dmypy and prospector without mypy locally and prospector with mypy in CI with the same config.

Currently the only alternative is to have duplicate configuration in both mypy.ini and .prospector.yml which is less than ideal.