Add pre-compiled JavaScript artifacts for Perspective package

[LukeSheard]

This PR does two things to enhance and improve the development workflow for apps running on the webpack plugin provided by @jpmorganchase/perspective:

1. Split the plugin itself into a separate package - this reduces overhead for installation when end users are only the UMD bundles.
2. The plugin now uses pre-compiled JS assets (via @babel/cli). This reduces the overhead on webpack for end users since it doesn't need to run babel itself on every build for the package anymore.
3. Webpack plugin now takes options for the various loaders needed during compilation so an end user can choose where the worker code will be written to (useful for caching).
4. Perspective Viewer + Plugins now exports a CJS module with it's dependencies externalised.

[LukeSheard]

@texodus any ideas why the /pr build failed for a screenshot change? Not sure how this change could affect that one individually.

https://travis-ci.org/jpmorganchase/perspective/jobs/478847521#L870

[texodus]

There seem to be some issues with this, notably that the example webpack project no longer compiles. Might I recommend fixing this example, which is both necessary and a good demonstration of what this refactor looks like from a consuming developer's standpoint?

[LukeSheard]

@texodus Updated the PR with all the comments we've discussed in person and on here. Still running into this issue when running the webpack example locally though:

[LukeSheard]

To reduce the scope creep of this PR I've raised #390, #391 and #392 and #393 which fix a lot of the initial issues I had with getting the webpack-plugin to work as expected.

To avoid conflicts once those are merged I can raise the final webpack-plugin PR.