Add simple health check (#37)

* Add simple health check

Useful to check health status of a running container using a `livenessProbe` in a
Kubernetes Pod.

* Change health endpoint to /-/healthy

nginx_exporter.go

@@ -193,6 +193,9 @@ func main() {
 			</body>
 			</html>`))
 	})
-
+	http.HandleFunc("/-/healthy", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(200)
+		w.Write([]byte("ok"))
+	})
 	log.Fatal(http.ListenAndServe(*listeningAddress, nil))
 }