Description: Sets or gets the callback function to be invoked before dismissing the notification [ if the invocation result === false , notification dismissal will be canceled ].

Default: undefined

 var canDismiss = false;
 var notification = alertify.success('You can dismiss this after 5 seconds!');
 notification.ondismiss = function(){ return canDismiss; };
 setTimeout(function(){ canDismiss = true;}, 5000);