Bat.lua: Adding naughty presets for customizable notifications

This commit adds more customization options to the naughty notifications of the battery widget.

It works like in the mpd widget: By adding two global preset tables which one can customize in the rc.lua.
* `bat_notification_low_preset` if battery charge level <= 15
* `bat_notification_critical_preset` if battery charge level <= 5

The old notification settings remain as the new preset default values. 
(Though I removed `position = "top_right"` and `ontop = true` as these are Naughty defaults anyway.)

I couldn't figure out how to commit wiki changes, but you can find my suggested wiki update [here](https://github.com/blueluke/lain/wiki/bat).