METAVAR, Improved help, Samples (Based on #187)

[p-ranav]

• Added metavars. If set, then will be used in help for the value placeholder for options, or the placeholder alone for regular positional arguments.
• Help option sections are indented by two spaces, and comma-separated. They now show metavar (this means that an option with value now shows as --option VALUE Description of Option instead of --option Description ....
• Usage section now enumerates options, instead of just [options]. Added ArgumentParser::usage() to retreive this string alone (in case you wanted to, say, print usage if arguments incorrectly passed).
• Not showing [default: ...] for implicit values. This seemed unusual. It might be better to rework the help system to recognise a placeholder (even python's %(default)s) rather than try to magically get this right.
• Added nargs to help, right after description of the argument
• Added samples for testing and demo

Example

#include <argparse/argparse.hpp>

int main(int argc, char *argv[]) {
  argparse::ArgumentParser program("main");
  program.add_argument("thing").help("Thing to use.").metavar("THING");
  program.add_argument("--member")
      .help("The alias for the member to pass to.")
      .metavar("ALIAS");
  program.add_argument("--verbose").default_value(false).implicit_value(true);

  program.add_description("Forward a thing to the next member.");
  program.add_epilog("Possible things include betingalw, chiz, and res.");

  program.parse_args(argc, argv);

  std::cout << program << std::endl;
}

foo@bar:/home/dev/$ ./build/samples/description_epilog_metavar --help
Usage: main [-h] [--member ALIAS] [--verbose] THING

Forward a thing to the next member.

Positional arguments:
  THING         	Thing to use. 

Optional arguments:
  -h, --help    	shows help message and exits 
  -v, --version 	prints version information and exits 
  --member ALIAS	The alias for the member to pass to. 
  --verbose     	

Possible things include betingalw, chiz, and res.