Factors argument parsing out into its own module

[xsdg]

This is a temporary step; the parser will eventually end up in bin/mkchromecast, since it defines the command-line interface.

[xsdg]

I tested this by doing steps similar to this (in debian, from the top-level mkchromecast/ directory):

$ mkdir /tmp/pyenv
$ python3 -m venv /tmp/pyenv
$ /tmp/pyenv/bin/pip install <mkchromecast dependencies>
$ python3 setup.py bdist_wheel
$ /tmp/pyenv/bin/pip install dist/mkchromecast-0.3.9-py3-none-any.whl
$ source /tmp/pyenv/bin/activate
$ mkchromecast --help  # To make sure options are listed as expected
$ mkchromecast -d  # To make sure options are used by mkchromecast as expected
$ deactivate

The options were listed as expected, and mkchromecast -d succeeded in listing my Chromecast device.