Module docstrings

[jdkandersson]

Pep257 discusses documenting functions, classes and other objects exported from a module should be documented in the module docstring. Add a linter that checks this:

# module.py

"""...

Functions:
    foo: ...

Classes:
    FooClass: ...

Constants:
    FOO: ...
"""

FOO = ...

def foo(): ...

class FooClass: ...

[jdkandersson]

Maybe support abbreviation Funcs and Clss?

[jdkandersson]

And also support Consts abbreviation?

[jdkandersson]

Private constants, functions and classes would be optional, indicated by a leading _, although all that are documented should actually exist in the module.

[jdkandersson]

Documenting constants would also be optional since many constants's name could be enough documentation for them