Add getter for positional argument's value field

[jhiolo]

I want to retrieve the positional args of a subcommand at the root level and to be able to use it like this:

parser := flags.NewNamedParser("blabla"), flags.HelpFlag)
root := parser.Command

cmdInstall.RegisterCmd(root)

parser.CommandHandler = func(commander flags.Commander, args []string) error {
	for _, arg := range parser.Active.Args() {
		log.Debugf("Positional arg %s", arg.Value())
	}
	return commander.Execute(args)
}

I think the easiest implementation with minimal impact is to use a getter.