Add support for Platform.io build targetted at ESP32 boards

[ajlennon]

Project is in platform.io folder. Open this and dependencies should auto-install. This uses ESP-IDF not Arduino framework.

• The example ESP32 board is the M5Stack Gray.
• No external PSRAM or flash needed. Build is about 800K
• A message file is loaded in Messages.c so you need an SD Card supported. Pinout might be different from the M5Stack in which case change the definitions in main.c
• It is probably possible to build the messages file into the code. This is left as an exercise for the user.
• I've run a pub/sub test here on it running on an ESP32 for a few hours and it seems quite reliable
• WiFi STA settings are also in main.c and will need changing. There's no clever provisioning in place atm

[ajlennon]

NOTE: I have signed the ECA but I do not believe the questions are correctly formulated. Question "C" in particular makes no sense to me