title: How do I count specific values across multiple columns in pandas
body: <p>I have the DataFrame</p>
  <pre><code>df = pd.DataFrame({
      'colA':['?',2,3,4,'?'],
      'colB':[1,2,'?',3,4],
      'colC':['?',2,3,4,5]
  })
  </code></pre>
  <p>I would like to get the count the the number of <code>'?'</code> in each column and return the following output -</p>
  <pre><code>colA - 2
  colB - 1
  colC - 1
  </code></pre>
  <p>Is there a way to return this output at once. Right now the only way I know how to do it is write a for loop for each column.</p>