added flake8-mutable

[vr050714]

Added flake8-mutable to Bugs section. The extension detects usage of mutable objects as default values for arguments like this.

def foo(x = []):
    return x

a = foo()
b = foo()  # gets reference to the same list

Note: In order to enable notifications the additional M class must be added to the selelct parameter in the settings.

Closes #136

[orsinium]

Thank you!

[DmytroLitvinov]

Thanks 👍