title: dplyr: filter based on another column
body: <p>Let's say that I have the following data and am interested in grabbing data by date where the type is "ts". Of course, there are dates where ts is not available, and I need to revert to the 'real' values for those dates.</p>
  
  <pre><code>dat = data.frame(dte = c("2011-01-01","2011-02-01","2011-03-01","2011-04-01","2011-05-01",
                           "2011-01-01","2011-02-01","2011-03-01"),
                   type = c("real","real","real","real","real","ts","ts","ts"),
                   value=rnorm(8))
  dat
  
  cpy = dat %&gt;% dplyr::filter(type == "ts") 
  
  cpy
  </code></pre>
  
  <p>How can something like that be done in dplyr.</p>
  
  <p>Expected output is:</p>
  
  <pre><code>dte            type    value
  "2011-01-01"   ts      ....
  "2011-02-01"   ts
  "2011-03-01"   ts  
  "2011-04-01"   real
  "2011-05-01"   real
  </code></pre>