Improve MQTT switch support.

lib/controller.js

@@ -156,7 +156,14 @@ class Controller {
         }
 
         // Convert the MQTT message to a Zigbee message.
-        const json = JSON.parse(message);
+        let json = null;
+        try {
+            json = JSON.parse(message);
+        } catch (e) {
+            // Cannot be parsed to JSON, assume state message.
+            json = {state: message.toString()};
+        }
+
         Object.keys(json).forEach((key) => {
             // Find converter for this key.
             const converter = mqtt2zigbee[key];