Add support for images in Shiny documents

This change adds a new flag, `copy_images`, to the HTML document base output format. When specified, it will scrape all of the images from the output document (using a regex), copy them to the output file's folder of supporting files, and rewrite the image references to point to the copies rather than the originals. 

This flag is then used when a Shiny document is run, so that it isn't necessary for Shiny to serve any resources outside the document's private library.