title: add more argument to summarise in dplyr
body: <p>my question is besides defining a function outside of summarise_each with multiple argument, is there another way to add the argument directly in the summarise_each?</p>
  
  <p>For example I want to get the mean without NAs.this way works </p>
  
  <pre><code>mean_fun=function(x)mean(x,na.rm=TRUE)
  AA_group=AA_new %&gt;% group_by(tractID)
  AA_group %&gt;% summarise_each(funs(mean_fun))
  </code></pre>
  
  <p>I am wondering whether there is a way to add <code>na.rm=TRUE</code> directly to <code>summarise_each</code>,such as <code>more_args</code> option?</p>
  
  <p>and also if I put mean_fun directly to summarise_each namely, </p>
  
  <pre><code>AA_group %&gt;% summarise_each(funs(function(x)mean(x,na.rm=TRUE)))
  </code></pre>
  
  <p>and the error is </p>
  
  <pre><code>expecting a single value
  </code></pre>
  
  <p>Does that mean that every time we want to use summarise_each, we have to define a function outside of that?</p>