Don't notify pre-releases

[Mte90]

I follow various projects that use CI to do pre-release of projects but I can get a lot of notifications, so I am wondering if it is possible to hide those releases cases.

[Mte90]

Looking at the github API the notification API doesn't inform about the releases.
So it should see if the notification is a release and check the status.

[freaktechnik]

This should indeed be possible. The extension tries to fetch the complete REST object associated with a notification. As such, it knows about all the details of a release (if it's allowed to). However, there is currently no filtering anywhere, so if any filtering were to be added it should, like #155, be implemented more generic and have actual UI in the options.