title: .str.contains returning actual found value instead of True or False
body: <p>I am using <code>str.contains</code> in my dataframe to see if a certain value is inside the values of a Series.</p>
  <p>Instead of the output being <code>True</code> or <code>False</code>, I want to see the actual value that I pass inside the contains.</p>
  <pre><code>A     B
  1   Fer
  2   Ger
  3   Tir    
  </code></pre>
  <p>My expected output:</p>
  <pre><code>A     B    C
  1   Fer   er
  2   Ger   er
  3   Tir  Nan 
  </code></pre>
  <p>Is there a built-in way to do this with pandas?</p>