Reset possibility for the WiFiManager

[philsson]

There is currently no way of getting back to blank settings. A proposal is to use the push button on the board to trigger this somehow. Desirably it would be a long press or pattern.
The call should be made to the only instance of "Settings" called "config" on its method resetWiFi().

After a reset the NodeMCU will start its own AP and be accessible on the IP 192.168.4.1 for configuration in accordance with the documentation for WiFiManager.
If the configured WiFi is not reachable / able to connect this will trigger the same configuration.