Mark command parameter required if there is no fallback
Mark command parameter required if there is no fallback

This change results in:

 - The "command" parameter being shown as <required> rather than
   [optional] when there is no fallback set with .nocommand()

 - It being an error (thus showing help) if no command is passed when
   one is expected, and there is no fallback set with .nocommand()

GerHobbelt/nomnom,matiassingers/nomnom,mozilla-b2g/nomnom,harthur/nomnom,douglasduteil/nomnom,Student007/nomnom,atlassian/nomnom