Add play_match function and command line interface for matches.

Future improvements might include listing of available brains, and a way to store game data easily.
I'm not sure we're ready yet for a parser - if we don't really have a way to load anything except the built-in random AI and the built-in human AI, there's not much utility here beyond what you'd get by just calling play_match(), right?
I'm hesitant to put this in place here because of the marginal complexity it adds for folks understanding the code.