Implement graceful shutdown

- Shutdown the HTTP server before exiting
- Persist data to store before exiting, if applicable

main.go

@@ -1,17 +1,37 @@
 package main
 
 import (
+	"log"
 	"os"
+	"os/signal"
+	"syscall"
 
 	"github.com/TwinProduction/gatus/config"
 	"github.com/TwinProduction/gatus/controller"
+	"github.com/TwinProduction/gatus/storage"
 	"github.com/TwinProduction/gatus/watchdog"
 )
 
 func main() {
 	cfg := loadConfiguration()
 	go watchdog.Monitor(cfg)
-	controller.Handle()
+	go controller.Handle()
+	// Wait for termination signal
+	sig := make(chan os.Signal, 1)
+	done := make(chan bool, 1)
+	signal.Notify(sig, os.Interrupt, os.Kill, syscall.SIGTERM)
+	go func() {
+		<-sig
+		log.Println("Received interruption signal, attempting to gracefully shut down")
+		controller.Shutdown()
+		err := storage.Get().Save()
+		if err != nil {
+			log.Println("Failed to save storage provider:", err.Error())
+		}
+		done <- true
+	}()
+	<-done
+	log.Println("Shutting down")
 }
 
 func loadConfiguration() *config.Config {