shinyShortcut - Creates an Executable Shortcut for Shiny Applications

Provides function shinyShortcut() that, when given the base directory of a shiny application, will produce an executable file that runs the shiny app directly in the user's default browser. Tested on both windows and unix machines. Inspired by and borrowing from <http://www.mango-solutions.com/wp/2017/03/shiny-based-tablet-or-desktop-app/>.