title: pandas DataFrame.to_html change negative numbers in html table as red
body: <p>I have simple pandas DataFrame which I need to convert to html table such like this one</p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame({
                  'Col1': ['20180817','20180816'],
                  'Col2': [3541, 2547],
                  'Col3': [-254, 147],
                  'Col4': ['-74.5%', '25.28%']
               })
  
  df_html = df.to_html(index=False, col_space=500)
  </code></pre>
  
  <p>but also need to change color of all negative numbers to red, e.g.</p>
  
  <pre><code>&lt;td&gt;-74.5%&lt;/td&gt;
  </code></pre>
  
  <p>Is there any simple way, how to do that? Thank you very much for any hint.</p>