Add runtime option for using pre-downloaded JS content rather than CDNs

[nathancarter]

This will require a build or install process that pre-downloads the latest CDN versions, and perhaps even does so regularly when used, to stay up-to-date. This will enable the tool to be used when not online. It may also make it so that notebooks can be reloaded from disk with visualizations automatically recreated without the user's needing to re-evaluate the cell, like they do now.