add rfkill kill switch support

[jlo62]

Hi, it would be great to have a osd when I toggle wifi/bluetooth.

This would simply need to run:

rfkill block [identifier]
rfkill unblock [identifier]
rfkill toggle [identifier]

[identifier] can be àll, wlan, bluetooth` or a number or string associated with a specific device.
[identifier] would be added as a command line argument.

As far as I know rfkill should be available for all users by default, so this wouldn't need root.

btw - great project, been looking for something like this for a while!

- Jan Luca