title: Add missing columns from different data.frame filled with 0
body: <p>I have the following situation:</p>
  
  <pre><code>df1
  a b c d
  1 2 3 4
  
  df2
  a c
  5 6
  </code></pre>
  
  <p>And the result I want is, to fill up the second data.frame with the missing columns from df1 and fill them with zeros. So the result should be:</p>
  
  <pre><code>df3
  a b c d
  5 0 6 0
  </code></pre>
  
  <p>The Data frames are quite big and that is why an automated way of doing this would be gerate.</p>