Docstring parser

[timcera]

Found this library https://github.com/rr-/docstring_parser for parsing docstrings. It auto detects the Sphinx, Numpy, and Google docstring styles. I think this simplifies mando a little bit and reduces the dependencies.

Does mean that the feature of specifying the long and short keyword arguments (for example, "--long" and "-l") has to be done in @arg. The mando docstring style was the only one that supported the capability of including in the docstring itself.

[rubik]

Hi Tim, thanks for another big contribution. It's definitely a good thing to offload the complicated parsing to a separate package.

1. Is this backward-compatible with existing Mando usage?
2. Can you address the failing tests on Travis? https://travis-ci.org/github/rubik/mando/builds/761532370

Thanks!