ArgumentBase.UsageText

[actualben]

I think it would be nice to be able to specify Usage text within positional argument definitions (in the same way we do with flags). Right now this seems like it could be possible with ArgumentBase.UsageText, but setting that doesn't seem to do what I want.

	cmd := &cli.Command{
		Arguments: []cli.Argument{
			&cli.StringArg{
				Name:  "target",
				UsageText: "network address of host to ping",
			},
			&cli.StringArg{
				Name:  "endpoint",
				UsageText: "url of endpoint to notify",
			},
		},
		Flags: []cli.Flag{
			&cli.IntFlag{
				Name:  "count",
				Value: 1,
				Usage: "`NUM` of pings to send",
			},
		},
		Action: DoSomething,
	}

With this code -h doesn't report the arguments at all. It would be nice if it printed something like:

NAME:
   pingreporter - ping the target, report the results to the endpoint

USAGE:
   pingreporter [global options] [arguments...]

GLOBAL ARGUMENTS:
   target    network address of host to ping
   endpoint  url of endpoint to notify

GLOBAL OPTIONS:
   --count NUM  NUM of ping probes to send (default: 1)
   --help, -h   show help

So, I'm wondering what's the intended purpose of ArgumentBase.UsageText, and would you be open to a PR that implements this kind of treatment of arguments?