Add number of returns validator

[Melevir]

If function has more that n returns in it's body, the plugin should raise error.
Max number of returns should be flake8 parameter.

[tripcher]

I'm ready to make the enhancement, but I have one question. Do I need to count returns in nested functions?
For example:

def decorator(func):
    def wrapper():
        print('example')
        return func()

    return wrapper

Does the decorator function have 1 return or 2?
For example the has_returns function in mr_proper counts returns in nested functions