Allow grouping subparsers together in the help output about argparse HOT 5 CLOSED

The basic idea is good, though I don't like adding yet another parameter to
ArgumentParser. I see two alternatives that I'd probably be okay with:

(1) Move the add_subparsers method into _ArgumentGroup. This would make putting
subcommands under a heading just like putting anything else under a heading, and
you'd write something like::

    commands_group = parser.add_argument_group(title='subcommands')
    subparsers = commands_group.add_subparsers()
    ...

(2) Teach the add_subparsers() method about title= and description= parameters, 
and
if either of these are present, create the corresponding argument group and add 
the
subparser action there instead of to the _positionals list. This would be a 
little
less pure, since normally only groups add new headings, but it might be more
practical because you could write something like:

    subparsers = parser.add_subparsers(title='subcommands')
    ...

What do you think?

I think I like (2) better, since (1) implies that you could have a separate 
subparser
for each group, which I don't think was the intention.  I guess there should 
only be
one subparser per parser, not group, right?

Here's a patch that implements option two.

Looks good, thanks for the patch. I'll try to get this committed this week.

Committed in r34. Thanks again for the patch!