title: Using starts_with in dplyr with a vector of partial column names
body: <p>I would like to use dplyr to select certain columns that match to a string vector.</p>
  
  <pre><code>one &lt;- seq(1:10)
  two &lt;- rnorm(10)
  three &lt;- runif(10, 1, 2)
  four &lt;- -10:-1
  
  df &lt;- data.frame(one, two, three, four)
  
  vars &lt;- c('on', 'thr')
  </code></pre>
  
  <p>I want to select only the columns in df whose titles start with'on' or 'thr':</p>
  
  <pre><code>dplyr::select_(df, starts_with(vars))
  </code></pre>
  
  <p>However, the above is not working.</p>