Allow running a command for notifications

[osa1]

Currently we use notify-rust for showing desktop notifications on mentions/private messages. This is not too useful when using tiny on a remote machine.

If we allow configuring command to run for sending notifications I could make the remote machine send me an email for notifications or SSH into my desktop and run notify-send etc.

[osa1]

If we do this I think we can remove libdbus dependency and just spawn notify-send for notifications. That program probably doesn't exist on OSX so we need to find an equivalent for the default notification command on OSX.