Add alias support to subcommands

I too would quite like to see the functionality from PR #4 included. This is based off of David's work, with Python 3 compatibility and a doc example.  If there are any additional changes required, just say the word.

Thanks,

James

PS. I'm opening this PR instead of commenting in #4 as it also allows you to hit the Green Button™ should the commit be acceptable, and also because GitHub will link the issues nicely.  I apologise if you would have preferred otherwise.