title: Place a grey box around chart title in ggplot2
body: <p>when we use the <code>facet</code> option in ggplot, we get a beautiful grey box around the headings (3,4,5).</p>
  
  <pre><code>library(ggplot2)
  
  data(mtcars)
  
  ggplot(mtcars, aes(cyl)) + geom_bar() + facet_wrap(~gear) + theme_bw()
  </code></pre>
  
  <p>How can we place a similiar grey box around the chart title when we do no use the <code>facet</code> option?</p>
  
  <pre><code>ggplot(mtcars, aes(cyl)) + geom_bar() + theme_bw() +
    ggtitle("How do you put a grey box around me??")
  </code></pre>