title: dpylr how to summarize given multilevel grouping
body: <p>I have a dataframe like </p>
  
  <pre><code>count  sample  type
  1  DLF002     a
  1  DLF002     a
  1  DLF002     b
  3  DLF001     a
  3  DLF001     b
  3  DLF001     b
  </code></pre>
  
  <p>and would like to make a summary given two factor (sample and type), so I would like to have a result like the following</p>
  
  <pre><code>count  sample  type
  2  DLF002     a
  1  DLF002     b
  3  DLF001     a
  6  DLF001     b
  </code></pre>
  
  <p>I have tried something like,</p>
  
  <pre><code>group_by(df, sample) %&gt;% summarize(sx = sum(count)),
  </code></pre>
  
  <p>but this does not into account the "type"; I wonder how I can make this two level grouping, using dpylr</p>