title: GROUP_CONCAT with dplyr or R
body: <p>I am having difficulty replicating the functionality of a typical SQL GROUP_CONCAT function in dplyr. I would also like to make sure the ordering inside the groups can be controlled. Ideally I want to use the hadleyverse/tidyverse but base R or other packages will work too. </p>
  
  <p>Example data:</p>
  
  <pre><code>ID    name
  1     apple
  1     orange
  2     orange
  3     orange
  3     apple
  </code></pre>
  
  <p>Desired output:</p>
  
  <pre><code>ID    name
  1     apple,orange
  2     orange
  3     apple,orange
  </code></pre>
  
  <p>Note that for ID=3, the ordering is in alpha order, not how the rows are ordered. I think this can probably be handled by doing an <code>arrange</code> first, but it would be nice to control inside the <code>summarise</code> statement or the like.</p>