WIP: Add basic support for imshow.

This is not ready, but I'm submitting early in case any others want to jump in and help improve it.

Current status: Nothing is broken. Image data is correctly encoded as base64 and included in the HTML. However, an svg:image object is not showing up on the page.

Thanks for this project, Jake. Amidst the storm of enticing plotting alternatives, this one is immediately useful.