Add voltage command

[scottleibrand]

In order to integrate with OpenAPS and allow us to safely shut down the Pi when voltage drops too low, it would be good to have a command-line tool that works like the voltage command from https://github.com/cjo20/EdisonVoltage, specifically in outputting json like:

root@edison-eb3 ~/myopenaps # ~/src/EdisonVoltage/voltage json batteryVoltage battery
{"batteryVoltage":3815, "battery":64}

It looks like this should be doable using the voltage library that's already used by openaps-menu (https://github.com/openaps/openaps-menu/tree/master/lib/voltage).

[bnielsen1965]

I just pushed a commit that adds in a unix socket that will be used to execute commands such as reading the battery voltage.

The commit also includes the read_voltage command and a bash script that will produce a backward compatible battery voltage response.

I.E.
root@raspberrypi:/home/pi/openaps-menu/scripts # ./getvoltage.sh
{"batteryVoltage":4080,"battery":96}

Try it out when you have a chance and if it works as expected the next step is to develop accurate battery percent readings.

[bnielsen1965]

The battery code now includes a configuration file to define the discharge curve and will show percentage of battery remaining.