Notification toggle command

[LordMZTE]

Desktop notifications can currently be configured in the config, but it does not seem possible to then disable notifications temporarily in a running iamb instance without modifying the config. It would be useful if we had a command such as :notifications {on,off,toggle} that could be used to temporarily turn on and off notifications when they're already configured in the config file.

[VAWVAW]

This could be implemented in combination with #510 to allow a :set command that could change any setting at runtime.