title: dplyr mutate, custom function and variable name as characters
body: <p>It's still not fully clear to me how I can pass certain expressions to dplyr.</p>
  
  <p>I'd like to use a user defined function within mutate and be able to pass it column names as characters. I tried a few things with interp{lazyeval} with no success.</p>
  
  <p>See the dummy example below.</p>
  
  <pre><code>library(dplyr)
  library(lazyeval)
  
  # Define custom function
  sumVar &lt;- function(x, y) { x + y }
  
  # Using bare column names (OK)
  iris %&gt;%
    mutate(newVar = sumVar(Petal.Length, Petal.Width))
  
  # Using characters for column names (does not work)
  iris %&gt;%
    mutate_(newVar = sumVar('Petal.Length', 'Petal.Width'))
  </code></pre>