Allow hiding arguments from help

[lolgab]

It is useful, when you want to deprecate a command line argument, to hide it from the instructions given by --help.
I'm trying to migrate an application using case-app and I'm missing this feature ( done with the @Hidden annotation in case-app

[morgen-peschke]

This would also be helpful if you're doing something like this (which allows the script to know what directory it lives in):

#!/bin/bash

DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" &>/dev/null && pwd)
exec amm "$0" --dir "$DIR" "$@"

!#

// Ammonite script follows

--dir isn't really relevant to the user, it's just plumbing, so hiding it would be ideal