Group and label option groups in the help screen

[natecook1000]

Option groups are currently just splatted into the rest of the declared arguments for a command. This is sometimes fine, but often it would be nice if option groups were separated and labeled, which would (a) help readability in complex commands and (b) help users understand which options carry over across multiple subcommands.

For example, for declarations like this:

struct Options: ParsableArguments {
    @Flag(help: "Display extra information while processing.")
    var verbose = false

    @Option(help: "The number of extra lines to show.")
    var extraLines = 0
}

struct Example: ParsableCommand {
    @OptionGroup(title: "Shared Options")
    var group: Options

    @Flag(help: "Parse as a binary file.")
    var parseAsBinary = false

    @Argument(help: "The input file.")
    var inputFile: String?
}

The help screen could look something like:

% example --help
USAGE: example [--verbose] [--extra-lines <extra-lines>] [--parse-as-binary] <input-file>

ARGUMENTS:
  <input-file>            The input file.

OPTIONS:
  --parse-as-binary       Parse as a binary file.

SHARED OPTIONS:
  --verbose               Display extra information while processing.
  --extra-lines <extra-lines>
                          The number of extra lines to show. (default: 0)

  -h, --help              Show help information.

[natecook1000]

It would probably also be helpful to be able to hide an entire option group from the help for a command.