title: Count the occurence of words in a list of all rows of dataframe
body: <p>I have a dataframe in which one of column has rows with list of values. I want to count the number of occurence of all the words inside the list among all rows.</p>
  
  <p>For ex: dataframe df</p>
  
  <pre><code>Column A         Column B
  animal            [cat, dog, tiger]
  place             [italy, china, japan]
  pets              [cat, dog]
  </code></pre>
  
  <p>Then I need result as:</p>
  
  <pre><code>cat : 2
  dog: 2
  tiger: 1 and so on
  </code></pre>