[Feature request] Printing help when using a subcommand without additional arguments

[leonav-unizar]

Use case

When writing a subcommand that requires further arguments that were not supplied, print a help message for the subcommand.

Example

argparse::ArgumentParser program("test");

argparse::ArgumentParser process_subcommand("subcom");
process_subcommand.add_description("One subcommand");
process_subcommand.show_help_on_failure(); // EXAMPLE function to be added

process_command.add_argument("action")
  .help("Action to perform")
  .choices("A", "B");

Then, when running the command, adding the --help would have the same effect as it wasn't there:

# With the help flag
./test subcom --help
Usage: test subcom [--help] [--version] [--action]

# Without the help flag
./test subcom
action: 1 argument(s) expected. 0 provided.
Usage: test subcom [--help] [--version] [--action]