title: Count occurrences of certain string in entire pandas dataframe
body: <p>I have following dataframe in pandas</p>
  
  <pre><code> C1      C2       C3
   10      a        b
   10      a        b
   ?       c        c
   ?       ?        b
   10      a        b
   10      ?        ?
  </code></pre>
  
  <p>I want to count the occurrences of <code>?</code> in all the columns</p>
  
  <p>My desired output is column wise sum of occurrences</p>