Adding support for batteries without energy_now and energy_full.

Apparently not all systems have these values in `/sys/class/power_supply`, so the battery meter doesn't work. In the ACPI source code they use `current_now` and `charge_now` to calculate time remaining if other values aren't available, so I just ported that calculation and set it up to use that as a fallback.

I don't actually have a system where `energy_now` and `energy_full` exist, so I'd suggest testing this code on such a system to make sure it still works as expected. I'm not 100% confident in my Perl grep skills :neutral_face:.