Add an optional maximum number of opened tabs

Hey guys,

I was in love with ST [ZenTabs plugin](https://github.com/travmik/ZenTabs).

The behavior is pretty simple: if there is more tabs than the maximum allowed, close the latest activated tab.

I just added this feature to atom/tabs. The PR comes with tests, just tell me if it isn't enough.

See it running:

![tabs](http://i.imgur.com/tjjvgpc.gif)