title: \text does not work in a matplotlib label
body: <p>I am using matplotlib together with latex labels for the axis, title and colorbar labels</p>
  
  <p>While it works really great most of the time, it has some issues when you have a formula using \text.</p>
  
  <p>One really simple example.</p>
  
  <pre><code>from matplotlib import pyplot as plt
  plt.plot([1,2,3])
  plt.title(r"$f_{\text{cor, r}}$")
  
  plt.show()
  </code></pre>
  
  <p>This will result in an error message like:</p>
  
  <pre><code>IPython/core/formatters.py:239: FormatterWarning: Exception in image/png formatter: 
  f_{\text{1cor, r}}
     ^
  Unknown symbol: \text (at char 3), (line:1, col:4)
    FormatterWarning,
  </code></pre>
  
  <p>Is there an easy way to use \text in there? </p>