Emit InstanceStartupEvent after starting server (#1410)

coremain/run.go

@@ -98,6 +98,9 @@ func Run() {
 		showVersion()
 	}
 
+	// Execute instantiation events
+	caddy.EmitEvent(caddy.InstanceStartupEvent, instance)
+
 	// Twiddle your thumbs
 	instance.Wait()
 }