Serve content of index.html if it exists; implement user-specified defaults (#272)

This change is largely a convenience for Shiny Server. It adds the following functionality:

- Serving the contents of `index.html` if it exists, as described in #272. Note that Shiny does serve the content inside the empty Shiny document, so the concerns in #272 re: having some Shiny scripts/styles in the header are not yet addressed.
- Permitting the user to specify which document will be served at the root (`/`) URL. This feature now gets a richer set of heuristics; the root URL document is chosen as the first of:
    - A single R Markdown file in the directory
    - A file in the directory named `index.Rmd`
    - A file in the directory named `index.html`