Lazy loading, shared packages

[bollwyvl]

Thanks for bqplot-gl!

As on a number of other issues, PRs, it would be lovely if this package:

• didn't load its heavy deps up-front, using e.g. exports: () => import('./widgets')
• hoisted heavy deps to sharedPackages, so they could potentially avoid loading multiple copies in the browser.
• synchronize the deps with the upstream, e.g. underscore vs lodash, version ranges of d3

As, like bqscales, it's likely the changes would be less invasive than on bqplot.