Option to treat mypy failures as warnings

[jaraco]

In jaraco/skeleton#143, I'm attempting to adopt more strict mypy settings for my projects. Because these checks are part of the test suite and CI checks, any failed check breaks the integration.

I'd like a way to treat mypy errors as non-blocking, to get a report of failures but not fail the check, similar to how warnings work in pytest. It would be nice if pytest-mypy could expose an option to do something similar.

I could see a number of ways to approach this need:

• Have errors emitted as actual warnings.
• Disable failures, but provide a separate report (similar to pytest-cov).
• Treat the failures as xfail.

Maybe there are other options too.

[dmtucker]

I've taken a stab at this in #180.
@jaraco could you take a look when you get a chance and see if it meets your needs?