align '--' in Python docstring arg list

Tweak to function recently added python-args-to-docstring.

Now it will look like this:

def my_func(foo, barbar=0, bazbazbaz = False)
    """

    Keyword Arguments:
    foo       --
    barbar    -- (default 0)
    bazbazbaz -- (default False)

    """
Merge pull request #8 from jferguson-gnubio/python_docstring_arg_tweak

align '--' in Python docstring arg list