title: Pandas: find most frequent values in columns of lists
body: <pre><code>    x    animal
  0   5    [dog, cat]
  1   6    [dog]
  2   8    [elephant]
  </code></pre>
  
  <p>I have dataframe like this. How can i find most frequent animals contained in all lists of column.</p>
  
  <p>Method value_counts() consider list as one element and i can't use it. </p>