Upgrade build process to Rollup

[bigtimebuddy]

Fixes #129

The goal of this refactor was to restructure the project to be a little bit more modern and support tree-shaking for bundlers like Webpack. That's only possible using ES modules. No logic was touched for the loading itself, just the docs and how the project is assembled and built.

Changes

• Dropped use of watchify/browserify and replaced with Rollup
• Generates CommonJS, ESM (bundlers-only) and IIFE (browser-only) builds
• Fixes issues with documentation properties not showing up
• Updated dependencies

Breaking Changes

• Renamed the bundled middleware functions and updated the README to describe usage
• Removes the squirrelly exports which mixed default and named exports (generally should do one or the other) The docs reflect the exports via the node module. In the browser everything is hung on the Loader global object.

[englercj]

This is pretty awesome, thanks for all this work @bigtimebuddy! I will give it a read through this weekend and see if we can't do a major release soon.

Thanks again for all the effort.

[bigtimebuddy]

No problem 👍 most of this follows a build pattern similar to the new PixiJS packages so it was pretty straightforward