suppressed error on linux from battery plugin

`battery_level_gauge` from battery plugin  gave errors on Linux machines without a battery component:

    ➲                                                                                      ⚡[..........]
    No support for device type: power_supply

 Fixed that.

Also, while at it, added a `fully_charged` function for OSX, which can be used to check if the battery is fully charged or not (which is the case when machine is plugged in and 100% charged).