Refactor file.path.ci, add find.file.ci and file.exists.ci
Add support for shinyAppDir with single-file app.R
shinyApp: use NULL default for ui and server

This is so that, when a single-file app returns a shinyApp with a blank
`ui` argument, Shiny will not error out, and instead it can search for
a www/ subdirectory.
Add single-file app example
Update missing-UI web page to include app.R
Detect showcase mode for single-file apps
Automatically display app.R file in showcase mode
Merge pull request #563 from rstudio/single-file2

Single-file app support