add postback payload description in message

message.go

@@ -130,12 +130,19 @@ type PostBack struct {
 	Recipient Recipient `json:"-"`
 	// Time is when the message was sent.
 	Time time.Time `json:"-"`
-	// PostBack ID
-	Payload string `json:"payload"`
+	// PostBack Payload
+	Payload PostBackPayload `json:"payload"`
 	// Optional referral info
 	Referral Referral `json:"referral"`
 }
 
+// PostBackPayload represents payload data in PostBack
+type PostBackPayload struct {
+	Title string `json:"title"`
+	Payload string `json:"payload"`
+	Mid string `json:"mid"`
+}
+
 type AccountLinking struct {
 	// Sender is who the message was sent from.
 	Sender Sender `json:"-"`