Idea: low batt warning

[KarlEffinger]

I run my Hani Mandl on a Heltec board via USB power supply. Sometimes I use a power bank as power source. That's why I thought about a litte soft- and hardware addition:

• use one of the A/D pins with an appropriate voltage divisor to measure the "5V" voltage
• test for the voltage every 60 seconds and
  • warn, if the voltage droppes below 4.5 V
  • close the servo and stop operation with appropriate screen message
    This would prevent running low on battery while the servo is open.