repr for notebooks

[jacob-rosenthal]

We can define a SlideData._repr_html_() method (or maybe SlideData._repr_jpg_()) which would let us do pretty outputs in JupyterNotebook. For example we could make this method display a thumbnail of the image by default, along with some text describing it. This would be nice for users since you could see the slide without having to call any methods.

This is lower priority but seems straightforward to implement

see: https://ipython.readthedocs.io/en/stable/config/integrating.html#rich-display