zpmlib.commands: move argparser setup function

We will need to import it when generating Sphinx documentation. We
cannot import it from the 'zpm' script, but we can import it from the
'zpmlib.commands' module.
tox: move requirements into doc/requirements.txt

This allows us to cleanly extend them in the future without touching
the tox config.
doc: use sphinx-argparse to include cmdline args

This makes it easy to include up-to-date command line synopsis in out
documentation.