title: Merge columns into row by one colunm value
body: <p>How do I merge rows by one columns value?
  Suppose, we've got df:</p>
  
  <pre><code>variable  value
  x1        a
  x2        a
  x1        b
  x2        b
  </code></pre>
  
  <p>And I need data frame like this:</p>
  
  <pre><code>variable  value1  value2
  x1        a       b
  x2        a       b
  </code></pre>