Allow positional arguments to be omitted from the help text

[bruceg]

I have been using argh to develop a new cargo command. Cargo passes the actual command being executed as the first command line argument. In order to capture (and ignore) that argument, I have an extra positional argument in the structure, followed by the real positional arguments. However, this first argument shows up in the command line help and is quite confusing as users should not actually enter it. It would be good to have a flag that omits a positional argument (and possibly others) from the generated help text.