Mandatory options aren't very useful

[jamesderlin]

This was raised by https://stackoverflow.com/q/67384823/.

Currently the presence of a mandatory option (added by #177) will cause ArgParser.parse to throw an exception whenever the option is omitted. I don't think that this is very useful in practice since it breaks --help.

#156 requested adding an equivalent of required options from Python's argparse module, but note that Python's argparse:

• Has a special action='help' mechanism for specifying an option that should print help.
• Adds a special handler for -h/--help by default.

In contrast, package:args's ArgParser currently is not opinionated about how help should be handled, and -h/--help is typically handled by an ordinary boolean flag.