Pym embedding

[freeman-lab]

This PR is an attempt to use pym to embed visualizations within notebooks instead of the current approach of getting html from the embed link.

The advantages would be:

• An easy way to specify a global target size for visualizations, similar the different sizing "contexts" in seaborn. This fixes the current problem that many visualizations end up unreasonably wide, and unless they have a very horizontal aspect ratio, they fall off the page.
• Streaming updates happen immediately within notebooks!

And by using pym we still get dynamic height adjustment. Seems like an overall win, but there may be disadvantages I'm missing.

This implementation doesn't quite work yet, due to a pym not defined error in the browser that I'm trying to debug, but hopefully it's close.

cc @mathisonian