title: plyr::ddply equivalent in dplyr
body: <p>I personally learned <code>plyr</code> prior to <code>dplyr</code>, and I'm trying to normalize my code into the <code>dplyr</code> syntax wherever possible, but I get stuck with the following use-case:</p>
  
  <pre><code>ddply(
      .data = somedataframe, 
      .variables = c('var1', 'var2'),
      .function = 
          function(thisdf){
              ...
          }
  )
  </code></pre>
  
  <p>Where the <code>...</code> inside the function call is some <em>arbitrarily complex modification of the dataframe</em>. Note that the choice of <code>ddply</code> versus <code>dlply</code> (or anyother <code>dxply</code>) is purely for illustration. Does a function within <code>dplyr</code> exists (call it <code>dplyr::f</code> for the moment), that could also take an arbitrary modification function? For example:</p>
  
  <pre><code>somedataframe %&gt;% 
      group_by(var1, var2) %&gt;% 
      dplyr::f(.function = function(thisdf){ ... })
  </code></pre>
  
  <p>In my investigation of this functionality, all the examples that I could find were extremely simple <code>summarise</code> implementations of <code>ddply</code>.</p>