title: convert list of strings to dummy variables with pandas
body: <p>if I have the following panda <code>DataFrame</code>:</p>
  
  <pre><code>pd.DataFrame(columns=['name', 'tags'], data=[
      ['Rob', ['a', 'c']],
      ['Erica', ['b', 'c']]
  ])
  </code></pre>
  
  <p>table:</p>
  
  <pre><code>Name   tags
  Rob    ['a', 'c']
  Erica  ['b', 'c']
  </code></pre>
  
  <p>How would I convert this into:</p>
  
  <pre><code>Name   tags_a  tags_b  tags_c
  Rob    1       0       1
  Erica  0       1       1
  </code></pre>
  
  <p>If each row could only have 1 tag I could do this with <code>pd.get_dummies(df, columns=['tags'])</code> but this doesn't work when <code>tags</code> is a <code>List</code>.</p>