Add support for defining the MQTT topic prefix that Zigbee2MQTT uses.

hapz2m.conf

@@ -11,4 +11,7 @@
 	"Server": "tcp://localhost:1883",
 	"Username": "hapz2m",
 	"Password": "my_s3cr3t_PasSw0rd"
+
+	// Topic prefix of the Z2M instance (optional)
+	// "TopicPrefix": "zigbee2mqtt/"
 }