title: Extra statistics with summarize_at in dplyr
body: <p>Is there a way to add extra statistics to a <code>summarize_at</code> call? For example</p>
  
  <pre><code>iris %&gt;% group_by(Species) %&gt;% summarise_at(vars(), funs(mean, sd))
  </code></pre>
  
  <p>will compute the means and standard deviations of 4 columns (giving a total of 8 columns). Suppose I also wanted to know how many rows were in each group. I.e., something like</p>
  
  <pre><code># Below is not valid syntax 
  iris %&gt;% 
    group_by(Species) %&gt;% 
    summarise_at(vars(), funs(mean, sd)) + summarise(n())
  </code></pre>
  
  <p>Given that the above does not work a kludge is</p>
  
  <pre><code>iris %&gt;% group_by(Species) %&gt;% summarise_at(vars(), funs(mean, sd, length))
  </code></pre>
  
  <p>which produces, in effect, 4 copies of the count column.</p>
  
  <p>Perhaps this is beyond what can be conveniently handled by <code>summarize_at</code> and friends?</p>