title: Dplyr: String on both sides of mutate function
body: <p>Given this dataframe:</p>
  
  <p><code>t1 &lt;- data.frame(x=c(1,2,3), y = c(2,3,4))</code></p>
  
  <p>I would like to get the output as in this function:</p>
  
  <p><code>t1 %&gt;% mutate_('y' = 'y * 2')</code></p>
  
  <p>But in such a way that I can define the input somewhere else:</p>
  
  <pre><code>t2 &lt;- 'y'
  t3 &lt;- 'y * 2'
  t1 %&gt;% mutate_(t2 = t3) # Should produce output as t1 %&gt;% mutate_('y' = 'y * 2')
  </code></pre>
  
  <p>Thanks for your help!</p>
  
  <p>Best,
  Jan</p>