Issue title: Indicator count for queued notifications
 Issue body: First, thanks for humane, we like a lot and it works great.

There are certain goings-on in our application where a number of humane notifications can get shown in a short period of time. It's good that they queue up and can all eventually be seen by clicking or waiting.

I had the idea that perhaps each notification could have some indicator of how many more notifications are queued up. That is, along with the text in the notification there could be a number saying how many more messages there are "behind" the current one.

Does that make sense?

Kind thanks again and in advance. Take care.

 Comments: 
 Comment 0: interesting idea, that's an alternative to stacking that i haven't considered yet and would be much less invasive to the code base, hmm.. care to prototype something in a PR?

 Comment 1: Thanks for the quick reply.

I've never looked at the source, but perhaps I will if I can steal the time away.

Regardless, thanks again and take care. 

 Comment 2: #60 may be a start. (:

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