This change lets you provide a title for option groups, which is used
when generating the help screen. Titled option groups, when they exist,
are placed between the ARGUMENTS and OPTIONS section of the help.
Multiple option groups with the same title are coalesced into a single
group.

For example, this command declaration:

    struct Extras: ParsableArguments {
      @Flag(help: "Print extra output while processing.")
      var verbose: Bool = false

      @Flag(help: "Include details no one asked for.")
      var oversharing: Bool = false
    }

    @main
    struct Example: ParsableCommand {
      @OptionGroup(title: "Extras")
      var extras: Extras

      @argument var name: String?
      @option var title: String?
    }

yields this help screen:

    USAGE: example [--verbose] [--oversharing] [<name>] [--title <title>]

    ARGUMENTS:
      <name>

    EXTRAS:
      --verbose               Print extra output while processing.
      --oversharing           Include details no one asked for.

    OPTIONS:
      --title <title>
      -h, --help              Show help information.