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))
 }