Adding to Opts API to make adding sub-command more friendly

[millergarym]

Allows for the following

var (
	rflg    = &types.Root{}
	cliBldr = opts.New(rflg).
		Name("wx").
		EmbedGlobalFlagSet().
		Complete()
)

func main() {
	cli := cliBldr.Parse()
	err := cli.Run()
	if err != nil {
		fmt.Fprintf(os.Stderr, "%[2]s\nError: %[1]v\n\n", err, cli.Selected().Help())
		os.Exit(1)
	}
}

func init() {
	// using End
	cliBldr.AddCommand(opts.New(&struct{}{}).Name("cli").
		AddCommand(opts.New(rename.NewRename(rflg)).Name("rename")).
		AddCommand(opts.New(newsubcmd.New(rflg)).Name("new_sub_command")).
		End())

	// using AddStruct and NewStruct
	cliBldr.AddStruct("a",
		opts.NewStruct("b",
			opts.NewStruct("c",
				opts.NewStruct("d",
					opts.New(xxx.NewXxx(rflg)),
					opts.New(xxy.NewXxy(rflg)).Name("3456789"),
				),
			),
		),
	)

	// Adding new sub-command to existing cmd structure
	cliBldr.AddStruct("a/b/c/d",
		opts.New(xxx.NewXxx(rflg)).Name("abcsdaf"),
	)

	// Replace a sub-command struct holding position with a runnable sub-command
	cliBldr.AddCommand(opts.New(&versionCmd{}).Name("a"))

	// Adding new sub-command to existing cmd structure
	cliBldr.AddStruct("a/bb/cc/dd",
		opts.New(xxy.NewXxy(rflg)),
	)
}