v1.2

• Added the possibility to unsubscibe (thanks to @MatasJu)
• Many improvement and optimisations.
  • Removed uses of std::function
  • Indentation fixes
  • Use #define instead of static const class members
  • String type was used too much while it was not necessary. Replaced most of them with uses of char arrays
  • Renamed some variables for better code reading
  • Better MQTT serial logs
  • Better wifi connection attempts handling, should solve the issue encontered by PiterKh (#2)
  • Better handling of received messages in mqttMessageReceivedCallback
  • Added comments to the exemple sketch