title: R: Merge of rows in same data table, concatenating certain columns
body: <p>I have my data table in R. I want to merge rows which have an identical <code>customerID</code>, and then concatenate the elements of other merged columns.</p>
  
  <p>I want to go from this:</p>
  
  <pre><code>   title  author customerID
  1 title1 author1          1
  2 title2 author2          2
  3 title3 author3          1
  </code></pre>
  
  <p>to this:</p>
  
  <pre><code>           title           author Group.1
  1 title1, title3 author1, author3       1
  2         title2          author2       2
  </code></pre>