title: DPlyr summarize variable arguments
body: <p>I want to calculate many different statistics using summarize. How can I do something like in the example below?</p>
  
  <p>Eg in this example, I would want to generate a table with counts for each month with the days that have a temperature less than 60,61... up to 90 degrees.</p>
  
  <pre><code>aq = airquality
  aq %&gt;% group_by(Month) %&gt;% summarize(num_days_60=sum(Temp&lt;60), num_days_61=sum(Temp&lt;61) .... num_days_90=sum(Temp&lt;90))
  </code></pre>
  
  <p>The output should look like this</p>
  
  <pre><code>Month   num_days_60 num_days_61 ... etc all the way up to 90 for example
  5       8           8
  6       0           0
  7       0           0
  8       0           0
  9       0           0
  </code></pre>