Low battery notifications

[lion90]

While having the tray icon rules, if I'm running something on fullscreen, the battery can run off with no previous alert.

Having a notification when the battery reaches a certain threshold could be useful.

[lion90]

At first I was thinking about an audible alert, but then I found this approach to show OS-level notifications, like other peripherals might do.

I developed a version of my own following Roliga's path. If you don't find it rude, I could make it more user-friendly (disabled as default + args-enabling it, adding it to the README) and PR it.

[centic9]

Yes, such a PR would be highly welcome! We can allow the limit to be set when the alert is triggered.