A simple SLCAN implementation for the ESP32
Requires arduino-CAN library by Sandeep Mistry https://github.com/sandeepmistry/arduino-CAN
Inspired by https://github.com/mintynet/teensy-slcan by mintynet
This example uses the CAN feather shield from SKPANG for Adafruit's ESP32 HUZZAH Feather http://skpang.co.uk/catalog/canbus-featherwing-for-esp32-p-1556.html