ENH: Support Sphinx Aliases for Images

[mmcky]

Currently the following does not render well

Embedded Image Handling
-----------------------

.. |mountain| image:: _static/hood.jpg
   :scale: 10 %

Here is an image that is inline and referenced using sphinx alias |mountain|.

as it get's rendered in the notebook as

• need to figure out how to determine if a directive is in the context of an alias and pass the object in the jupyter writer
• update tests/image.rst with the test case