MP3 Player U2.1 Modular

Required libraries:
- ProjectUNION Core 0.3.1
- ProjectUNION MQTT 0.3.1
- PubSubClient
- DFRobotDFPlayerMini

This project must not require:
- ProjectUNION DCC
- NmraDcc

Test:
1. Compile and note program and RAM size.
2. Upload and test WiFi/web configuration.
3. Test MQTT connection.
4. Test Play, Stop, Pause, Volume and Song Number.
5. Test DFPlayer Mini and OTA.
