title: count total number of list elements in pandas column
body: <p>I have a pandas dataframe <code>A</code> with column <code>keywords</code> as 
  (here Im showing only 4 rows but in actual there are millions) :-</p>
  
  <pre><code> keywords
   ['loans','mercedez','bugatti']
   ['trump','usa']
   ['galaxy','7s','canon','macbook']
   ['beiber','spiderman','marvels','ironmen']
  </code></pre>
  
  <p>I want to sum total number of list elements in column <code>keywords</code> and store it into some variable. Something like </p>
  
  <pre><code>total_sum=elements in keywords[0]+elements in keywords[1]+elements in 
            keywords[2]+elements in keywords[3]
  
  total_sum=3+2+4+4
  total_sum=13
  </code></pre>
  
  <p>How I can do it in pandas?</p>