Ability to add custom transforms

[wadethestealth]

📝 Summary

With the new outputs allowing new transforms (previously mimetypes), we should allow plugins to build there own transforms and add them in.

🏁 Motivation

This could allow for custom component results without the need for kernel interception. It will also allow for adding new transform types hydrogen doesn't support.

• See No display of Vpython objects. #1556, Can't display the Map items in Atom with hydrogen, python and folium  #1069, Custom executables for display.Markdown [feature request] #1630

🏃 Approach

Our Plugin API could take in a react class/function and add it to supportedMedia for rendering.

⚓ Drawbacks

Unsure at what limitations there are yet, and I wonder if result rendering would slow.