battery_level: Add custom notification format
battery_level: Add custom notification format

The user can now choose how the notification is formatted when they
click on the battery_level module. Also, one can differentiate the
format based on the status of the alimentation (plugged or not)

Spirotot/py3status,ultrabug/py3status,ultrabug/py3status,Shir0kamii/py3status,valdur55/py3status,tobes/py3status,guiniol/py3status,vvoland/py3status,valdur55/py3status,valdur55/py3status,guiniol/py3status,Andrwe/py3status,alexoneill/py3status,Andrwe/py3status,docwalter/py3status,tobes/py3status,ultrabug/py3status