title: adding syntax highlighting to Jupyter notebook cell magic
body: <p>I'm trying to figure out how to activate CodeMirror syntax highlighting for a CodeMirror-supported language (cypher) within a cell for a custom Jupyter cell magic (<code>%%mymagic</code>). The magic isn't associated with a special kernel - it just runs Python commands that process the string entered into the cell that I want to highlight. From what I can tell, this ostensibly can be done using something like</p>
  
  <pre><code>from notebook.services.config.manager import ConfigManager
  cm = ConfigManager()
  cm.update('notebook', {'CodeCell': {'highlight_modes': {'magic_cypher': {'reg': '^%%mymagic'}}}})
  </code></pre>
  
  <p>within the class that implements the magic.
  I can't seem to get this to work, however; no change in highlighting occurs when I enter stuff in a cell that starts with <code>%%mymagic</code>. Is the above approach accurate? Does 'magic_cypher' need to have a specific format? Does the magic need to somehow specify the MIME type CodeMirror associates with the desired highlighting language? I'm using notebook 5.0.0, jupyter_core 4.3.0, and python 2.7.13.</p>