title: Use variable names in functions of dplyr
body: <p>I want to use variable names as strings in functions of <code>dplyr</code>. See the example below:</p>
  
  <pre><code>df &lt;- data.frame( 
        color = c("blue", "black", "blue", "blue", "black"), 
        value = 1:5)
  filter(df, color == "blue")
  </code></pre>
  
  <p>It works perfectly, but I would like to refer to <code>color</code> by string, something like this:</p>
  
  <pre><code>var &lt;- "color"
  filter(df, this_probably_should_be_a_function(var) == "blue").
  </code></pre>
  
  <p>I would be happy, to do this by any means and super-happy to make use of easy-to-read <code>dplyr</code> syntax.</p>