Add completion to "help" command arguments

[stalowyjez]

It might be not that easy to implement, but I think it's a nice improvement

[dextero]

You mean making the topic argument autocomplete available commands? That's a great suggestion!

To make this work, it's necessary to make the completer access the Cmd object itself, and self does not exist yet at the point of defining a class method. With the current method of handling completions, I think it would be necessary to set up do_help dynamically when Cmd is constructed, but it should be doable.