Add NotifierChan helper

[slok]

Add helper to get notifiers from string channels.

There are times where a simple channel is enough for the Notifier, for example, an HTTP handler:

reloadManager := reload.NewManager()

// Add reload notifier.
reloadChan := make(chan string)
reloadManager.On(reload.NotifierChan(reloadChan))

// On HTTP request to `/-/reload` trigger reload.
mux := http.NewServeMux()
mux.Handle("/-/reload", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
  reloadChan <- "http"
}))