UPS support

Hi, 

Thanks for great tmux plug-in!
It might be great to have UPS support. On my workstation it's reported as ups device

```
>> upower -e    
/org/freedesktop/UPower/devices/ups_hiddev0
/org/freedesktop/UPower/devices/mouse_0003
/org/freedesktop/UPower/devices/mouse_hidpp_battery_0
/org/freedesktop/UPower/devices/DisplayDevice
```

Thank you in advance!