Enable help for positional arguments

[ewels]

Click deliberately does not support help for positional arguments.

I think it would be good to support this if people want, it is a bit more relevant with the rich-click tabular output.

To make this work, we need to enable an additional argument for the click.argument() function though, which is a little involved. Others have asked for this feature and some have done their own implementations, see https://github.com/pallets/click/issues/587#issuecomment-410097642

Should be able to recreate something like this in rich-click to add support for argument help.