Introducing a notification when a proxied host becomes unhealthy

### What should we add or change to make your life better?
i already tried adding a "OnUnhealthy" delegate to "ActiveHealthCheckConfig" in this branch: https://github.com/mdrollo/reverse-proxy/tree/dev
(There are some issues in the sample project, but generally it's working)

### Why is this important to you?
using "OnUnhealthy", i can integrate the Reverse Proxy deeper with our WebApi, so it can takte actions when an instance is blocking (restarting it, spawning new instances etc)