Reporting status during the update process

[nonsintetic]

It would be very useful to have a way of getting info about what's going on during the update process outside of the library. This would let you, for example, publish some messages or errors to MQTT or BLE while the update is going on.

Think a simple way to do this would be to have a callback function that gets passed on initialization that then gets called whenever something happens during the update process.

Things that could be useful to track in my opinion: errors, download status for firmware file, start of the actual update process once the file is downloaded, result (success/fail).