Added shiny_demo function.

The shiny_demo function is designed to make it easier to run Shiny apps from R packages. It works similar to the demo function. Package developers can just put Shiny apps in the inst/shiny directory and this function will find the app automatically. Call shiny_demo() will print the available topics/apps. shiny_demo('topic') will run that Shiny app. The optional package parameter will help in cases where two packages have Shiny apps with the same name.