title: Combine multiple observations in R
body: <p>I have a flat file of tweets and would like to aggregate their properties by user. </p>
  
  <p>e.g.</p>
  
  <pre><code>user1, hashtag1, hashtag2 
  user1, hashtag3, hashtag4 
  user2, hashtag5, hashtag6 
  user2, hashtag7, hashtag8
  </code></pre>
  
  <p>Which I would like to transform into:</p>
  
  <pre><code>user1, hashtag1, hashtag2, hashtag3, hashtag4
  user2, hashtag5, hashtag6, hashtag7, hashtag8 
  </code></pre>
  
  <p>Is there an elegant ways to do this?</p>