title: Modify optparse help output with extra argument
body: <p>When using Python optparse, is there a standard way to have the standard <code>--help</code> option take an optional argument, and to produce different output than the standard auto-generated one?</p>
  <p>E.g. have <code>my_tool --help</code> show the default auto-generated output, but have <code>my_tool --help=topic</code> show custom help output for a specific topic only?</p>