Use a real argument parser

[dgw]

I've been avoiding work on #5 and #8 (and anything else related to arg parsing) for ages because it's just so annoying to deal in regex. So I'm making a decision: This module should use a real argument parser.

Since argparse is included in Python's standard library, that's probably the way to go. It looks like splitting the input into a list of strings and passing it to the parser will work (according to docs.python.org), and as long as parsing arbitrary argument lists is possible (since sys.argv is irrelevant to this application), everything else is just a matter of reading the docs.