Display battery voltage, shut off.

[MartinMueller2003]

Discussed in #456

^{Originally posted by perigalacticon January 5, 2022}
I have been using espixelstick at remote locations using battery power for the lights and esp. Often the battery will go low without my knowledge and the display goes bad before I catch it. Would it be possible to have espixelstick measure and display the supply voltage for the lights and the esp, and have some options for actions to take if it goes below a threshold, such as deactivate output for a start, or notify the owner somehow. I personally would find it invaluable as I've found battery life depends on many conditions that are difficult to account for.