title: Find last column matching pattern in each row
body: <p>I have a dataframe with several columns of 'actions'. How can I find the last action matching a pattern and return its column index or label?</p>
  
  <p>My data:</p>
  
  <pre><code>name    action_1    action_2    action_3
  bill    referred    referred    
  bob     introduced  referred    referred
  mary    introduced      
  june    introduced  referred    
  dale    referred        
  donna   introduced
  </code></pre>
  
  <p>What I want:</p>
  
  <pre><code>name    action_1    action_2    action_3    last_referred
  bill    referred    referred                action_2
  bob     introduced  referred    referred    action_3
  mary    introduced                          NA
  june    introduced  referred                action_2
  dale    referred                            action_1
  donna   introduced                          NA
  </code></pre>