title: Count how many times a column contains a certain value in Pandas
body: <p>Let's say my dataframe looks like this:</p>
  
  <pre><code>   column_name
  1  book
  2  fish
  3  icecream|book
  4  fish
  5  campfire|book
  </code></pre>
  
  <p>Now, if I use <code>df['column_name'].value_counts()</code> it will tell me <code>fish</code> is the most frequent value.</p>
  
  <p>However, I want <code>book</code> to be returned, since row 1, 3 and 5 contain the word 'book'. </p>
  
  <p>I know <code>.value_counts()</code> recognizes <code>icecream|book</code> as one value, but is there a way I can determine the most frequent value by counting the amount of times each column cell CONTAINS a certain value, so that 'book' will the most frequent value?</p>