Enhancement: remove previous notifications

[raiseandfall]

Hey there, first of all, great component ! thanks for the hard work.

I'd like to propose one enhancement: the ability to remove existing notifications before adding a new one.
Basically the component would only allow one notification at the time. Right now I can add several notifications and they stack on top of each other.
The way we could do it is restrict the number of notifications visible in the same time. If the number goes over the limit, the oldest notification would be removed, until reaching the limit again.

I'm thinking about adding one setting option: maxVisibleNotifications
This could be a parameter in the provider:

app.config(['notificationsConfigProvider', function (notificationsConfigProvider) {
    // max visible notifications
    notificationsConfigProvider.setMaxVisibleNotifications(1);
}]);

I'm currently too busy to take care of it & pull a request, but I will once I can. I just wanted to throw the idea out there.

Thanks