title: Shiny: What is the option setting to display in the console the messages between server and ui
body: <p>I remember having used an option setting that enable to print in the R console the messages between the server and ui while a shiny app is running.
  This option is really useful to understand what is happening in the app.
  However I cant remember the name of that option and can not find it anymore in the documentation.
  Can someone can tell what is that option ?
  Thanks!</p>