Check for consistency of implicit and explicit return values

[FichteFoll]

About a year ago, a recommendation about consistency with implicit and explicit return values was added: https://hg.python.org/peps/comparison/dcca553d365d/pep-0008.txt

It would be nice if pydocstyle could check for this. I don't know parsing is performed, but I believe this to be possible.

Due to the controversy of this recommendation and it most likely not being followed in a lot of code bases, it should probably be disabled by default.

[FichteFoll]

Same as with #192, I was at the wrong repo and didn't find the existing issue for it: PyCQA/pycodestyle#399.