Slack webhook options for more selective notifications
We like the Slack webhook, but it tends to be very chatty, even given the current configuration options for it. There are two cases we wish the webhook supported:
* Disabling notifications for debugging symbol package pushes. Since we usually push symbols with our internal packages, this results in two messages each time a package is pushed.
* Filtering between release and pre-release packages for notifications. We'd like to have release packages notify a different channel so the people on our team that only care about released updates can be notified. Today, they just mute the whole channel because the pre-release packages create too much noise.