Feature request: required_or, needs_or

Thought it might be nice to add a couple features 'required_or' and 
'needs_or'.

Each will let the user define one and only one option from a set of 
choices. A real world example may be that your program accepts a userid 
or a username. One but not both of the options are required to run 
successfully.

Valid:

    myprog.pl --userid 5
    myprog.pl --username foo

Invalid:

    myprog.pl --userid 5 --username bar