title: Generate N-Grams from strings with pandas
body: <p>I have a DataFrame <code>df</code> like this:  </p>
  
  <pre><code>Pattern    String                                       
  101        hi, how are you?
  104        what are you doing?
  108        Python is good to learn.
  </code></pre>
  
  <p>I want to create ngrams for String Column.
  I've create unigram using <code>split()</code> and <code>stack()</code></p>
  
  <pre><code>new= df.String.str.split(expand=True).stack()
  </code></pre>
  
  <p>However, I want to create ngrams (bi, tri, quad etc)</p>