title: changing outlier rule from IQR to standard devations in geom_boxplot
body: <p>I know the default setting for <code>geom_boxplot()</code> is:</p>
  
  <ul>
  <li>Q3 + 1.5 * IQR </li>
  <li>Q1 - 1.5 * IQR</li>
  </ul>
  
  <p>but I wanted to do 4 standard deviations from the mean:</p>
  
  <ul>
  <li>MEAN + 4 * SD</li>
  <li>MEAN - 4 * SD</li>
  </ul>
  
  <p>Is this possible to do in ggplot2?  If not, what is the alternative?</p>
  
  <p>I saw a post that asked about changing to different IQR, but I am specifically interested in changing to standard deviation.</p>