Handle MathML output

[takluyver]

Follow on from #1565.

The mimetype for MathML appears to be application/mathml+xml (source).

[gnestor]

@takluyver I'd be happy to look into this. A couple questions:

• Where are mime types defined in this repo?
• Where are handlers for mime types defined?

[takluyver]

This file should be the most relevant one: https://github.com/jupyter/notebook/blob/master/notebook/static/notebook/js/outputarea.js

[gnestor]

Thanks @takluyver! Here is an initial implementation #1672.

[gnestor]

Merged with #1672