Display custom command usage when it has no step

[stoned]

what

• When a custom command without steps is invoked without subcommand display its usage message.

why

• When custom command without steps is invoked without subcommand, 'atmos' outputs nothing and simply exits. It could be helpful to display a usage message and exit without error, like when it is invoked without any argument (like 'atmos') or a toplevel command without required argument is invoked without a subcommand (like 'atmos aws' or 'atmos describe')

[aknysh]

thanks again @stoned