* Add support for batteries outputting Time to empty on linux
The battery in the pinebook pro does not output the remaining charge
in Wh or mAh, so i3 cannot calculate the time remaining. However, it
does directly output the number of minutes remaining on in
POWER_SUPPLY_TIME_TO_EMPTY. This adds support for reading this field
and converting it to seconds_remaining.
* Add testcase for POWER_SUPPLY_TIME_TO_EMTY_NOW
