Add quiet option to runApp

When run with `runApp(quiet = TRUE)`, this quiets the "listening on port xxxx" message. This is useful when Shiny apps are run by other processes (like ggvis), where the details of running the Shiny app aren't important to the user. The alternative, using `suppressMessages(runApp())`, is a too heavy-handed since it blocks all messages generated by the app.

It would also be nice to be able to quiet the output from `addResourcePath()`, but doing so is less straightforward. Any input on how best to do that?