The functionality for no_args_is_help, available on MultiCommands, can
easily be added on Commands as well. The result is not much more code,
and a more consistent interface between MultiCommands and Commands.
It's also nice to be able to write a click command `foo` and then just
hit `foo` for help, regardless of whether its a Group or Command.

closes pallets#1156

sirosen deleted the no-args-help-command branch

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