Improve dev experience with custom ipywidgets

[peytondmurray]

Currently, ipywidgets_bokeh fetches the js it needs to render the frontend of a widget from a configurable CDN that defaults to unpkg. This is fine if the ipywidget is published and easily available from somewhere, but if a user is developing a widget locally, they have to intercept the web requests made by ipywidgets_bokeh and serve the appropriate bundle. It isn't at all obvious for someone who isn't extremely comfortable with both ipywidgets and ipywidgets_bokeh why this needs to be done and what they need to do to get around it.

This issue is meant to start a discussion around what can be done to make the barrier to entry smaller here.