Add hook for instance startup (#1888)

Provides a new hook for plugins as a means to provide the current caddy.Instance when starting or restarting.

caddy.go

@@ -213,6 +213,9 @@ func (i *Instance) Restart(newCaddyfile Input) (*Instance, error) {
 	}
 	i.Stop()
 
+	// Execute instantiation events
+	EmitEvent(InstanceStartupEvent, newInst)
+
 	log.Println("[INFO] Reloading complete")
 
 	return newInst, nil

caddy/caddymain/run.go

@@ -140,6 +140,9 @@ func Run() {
 		mustLogFatalf("%v", err)
 	}
 
+	// Execute instantiation events
+	caddy.EmitEvent(caddy.InstanceStartupEvent, instance)
+
 	// Twiddle your thumbs
 	instance.Wait()
 }

plugins.go

@@ -236,6 +236,7 @@ const (
 	StartupEvent         EventName = "startup"
 	ShutdownEvent        EventName = "shutdown"
 	CertRenewEvent       EventName = "certrenew"
+	InstanceStartupEvent EventName = "instancestartup"
 )
 
 // EventHook is a type which holds information about a startup hook plugin.