title: Adding an image to title slide using slidify
body: <p>I am using the latest version of Rstudio (and knitr) and I have installed the development version of slidify from github. Slidify allows you to go straight from Rmarkdown to html5slides with 1 click.</p>
  
  <p>My issue is that I would like to add an image to the title slide, but using the normal Rmarkdown syntax does not work.  I think this is because the first slide is specified as a YAML. </p>
  
  <pre><code>title       : 
  subtitle    : 
  author      : 
  job         : 
  framework   : io2012        # {io2012, html5slides, shower, dzslides, ...}
  highlighter : highlight.js  # {highlight.js, prettify, highlight}
  hitheme     : tomorrow      # 
  widgets     : []            # {mathjax, quiz, bootstrap}
  mode        : selfcontained # {standalone, draft}
  </code></pre>
  
  <p>If I add anything to this first slide specification, I get an error in the function yaml.load() which parses the YAML.  Is there a way to specify a background image using YAML?</p>