etcdctl: add extended as output format

extended wasn't documented in the help as one of the output formats, fix
this!

etcdctl/main.go

@@ -30,7 +30,7 @@ func main() {
 	app.Flags = []cli.Flag{
 		cli.BoolFlag{Name: "debug", Usage: "output cURL commands which can be used to reproduce the request"},
 		cli.BoolFlag{Name: "no-sync", Usage: "don't synchronize cluster information before sending request"},
-		cli.StringFlag{Name: "output, o", Value: "simple", Usage: "output response in the given format (`simple` or `json`)"},
+		cli.StringFlag{Name: "output, o", Value: "simple", Usage: "output response in the given format (`simple`, `extended` or `json`)"},
 		cli.StringFlag{Name: "peers, C", Value: "", Usage: "a comma-delimited list of machine addresses in the cluster (default: \"127.0.0.1:4001,127.0.0.1:2379\")"},
 		cli.StringFlag{Name: "cert-file", Value: "", Usage: "identify HTTPS client using this SSL certificate file"},
 		cli.StringFlag{Name: "key-file", Value: "", Usage: "identify HTTPS client using this SSL key file"},