Throttle/Group multiple notifications sent from the same app

It's not unusual to receive a few notifications at the same time from a Slack bot, for example. Instead of flooding the screen with notifications, it would probably make sense to group them together and just send one notification like:

**Slack**
10 new notifications

We're currently doing something similar in Mail when multiple emails come in at the same time. Making this a feature of the notification server could mean that you'd only get one notification on screen but we could send all the of the individual notifications to the indicator which would be nice. This would resolve the issue where you can end up with something in the indicator that looks like:

**Mail** now
5 unread messages

**Mail** 3pm
New message from Cassidy

**Mail** 2pm
3 unread messages