Add WebhookID member to message property (#607)

* track membercount on memberAdd and leave

* requested changes

* add WebhookID member to message

message.go

@@ -82,6 +82,9 @@ type Message struct {
 
 	// The type of the message.
 	Type MessageType `json:"type"`
+
+	// The webhook ID of the message, if it was generated by a webhook
+	WebhookID string `json:"webhook_id"`
 }
 
 // File stores info about files you e.g. send in messages.