Implemented 'Mandatory flags' per issue #44

Implemented 'Mandatory flags' based on the thread:
https://github.com/visionmedia/commander.js/issues/44.

Similar to the fix from this pull request:
https://github.com/visionmedia/commander.js/pull/70, but is additive
and does not break the existing semantics of 'required' arguments.

If this pull request is accepted I'll also update the docs : )