Hidden commands

Good day!

I hacked together the ability to suppress the help output of commands by adding a new `Command.prototype.hidden` function.  I find this particularly useful for removing the somewhat out of place asterisk for the unmatched command event handler, but I recognize there may be some use cases where someone would want to provide that information.  Consequently, I went with a more generic solution, and someone may just have some other user for it.

Please let me know if you would like me to make any additional changes or clean anything up.  Thanks!