push arg-parsing into argparse

don't bother with sys.argv directly
add --port option
restore pick_a_port() behind --port, WEBDIFF_PORT

nice to allow users to not have:

1. any cfg set up and still get a port that works, and
2. not have to have 8080 free necessarily
Merge pull request #18 from ryan-williams/port

use argparse, add --port option