Extended http_server to provide new endpoints:

/_mapping.txt, /player.json, /playlist_manager.json, /controller.json and /position.json to get the matching data as well as /cmd to send commands to.

src/controller.cpp

@@ -360,6 +360,8 @@ String Controller::json() {
 	JsonObject rfid = json.createNestedObject("last_rfid");
 	rfid["uid"] = _last_rfid_uid;
 	rfid["data"] = _last_rfid_data;
+	json["uptime"] = millis() / 1000;
+	json["free_heap"] = ESP.getFreeHeap();
 	return json.as<String>();
 }