Calling shtuff without args blows up

[James LaChance]

This article suggests having shtuff without any args to provide the help message. I agree with it, and I think we should go ahead and implement all the ways they suggest in showing the help message.

# list all commands
$ mycli
$ mycli --help
$ mycli help
$ mycli -h

# get help for subcommand
$ mycli subcommand --help
$ mycli subcommand -h