Added support for single app.R deployments as supported in shiny 0.10.2.

The one caveat here is that we don't gracefully handle the scenario in which an app.R file is found but the version of Shiny is not sufficiently updated to support the app.R model. If that occurs, shiny will produce an error about the UI not being found in `www/` nor `ui.R`

If we're ok with that, I'll merge this in and get it in Pro. If we want to explicitly check the version before trying to use this feature, then we'll need to bite off a larger chunk of work to make this a full-fledged third mode of operating Shiny so we can check the package versions within R like we do with Rmd.

Thoughts @jcheng5 / @wch ?