Add battery sensing (mostly) for TBEAM0.7

However, disabled until someone with suitable hardware can test and report
back.

@slavino and @tschundler would you be willing to try it with your boards?

You'll need to uncomment the following line in configuration.h

// #define BATTERY_PIN 35 // A battery voltage measurement pin, voltage divider connected here to measure battery voltage

src/main.cpp

@@ -210,13 +210,11 @@ void setup()
     esp32Setup();
 #endif
 
-#ifdef TBEAM_V10
     // Currently only the tbeam has a PMU
     power = new Power();
     power->setup();
     power->setStatusHandler(powerStatus);
     powerStatus->observe(&power->newStatus);
-#endif
 
 #ifdef NRF52_SERIES
     nrf52Setup();