Subcommand support

[arendsee]

It would be neat if argparse supported subcommands.

For example,

parser <- ArgumentParser()
subparser <- parser$add_subparsers()
foo_parser <- subparser$add_parser('foo', help="foo subcommand")
bar_parser <- subparser$add_parser('bar', help="bar subcommand")
foo_parser$add_argument('x', help="foo's x")
bar_parser$add_argument('x', help="bar's x")

The subcommand structure is really powerful for large command line programs (e.g. git) and is pretty easy to use in python's argparse.

[trevorld]

Subcommand support would be pretty cool however seems kind of complicated to implement. Adding this will be a low priority for me.

[trevorld]

Should do #25 before this.