command: replace optimist with commander

I couldn't work out how to have [optimist][1] include a flag in the --help output without giving it a default value. [Commander][2] is easier to use: it doesn't provide umpteen ways to specify things, and its documentation is better.


[1]: https://github.com/substack/node-optimist
[2]: https://github.com/visionmedia/commander.js