Add option to retain MQTT messages.

data/configuration.yaml

@@ -2,6 +2,7 @@ default:
   mqtt: 
     server: "mqtt://localhost"
     base_topic: xiaomi
+    retain: false
   serial: 
     port: /dev/ttyACM0
   allowJoin: true

index.js

@@ -190,7 +190,7 @@ function mqttPublish(topic, payload) {
     }
 
     logger.info(`MQTT publish, topic: '${topic}', payload: '${payload}'`);
-    client.publish(topic, payload);
+    client.publish(topic, payload, {retain: settings.mqtt.retain});
 }
 
 function writeConfig() {