Initial commit.
  * New document type html_report.
      * Automatically uses prism.js for syntax highlighting
      * Wraps images with <figure /> tags.
      * Uses the printr library to format all knitr output as HTML
        tables.
      * Sets the default output type to SVG (which has good enough
        browser support and sharper graphics than PNG).
  * addImage function adds different images depending on the output
    format.
  * include_script outputs an external file and then sets it up for
    the appropriate syntax highlighting