Make es6-promise polyfill optional for consumers of npm package

[alanpearce]

Libraries should not provide their own polyfills. I can see that some have already been removed from this package in the past, so I assume that's already well-understood here.

In order to keep a smooth upgrade process, I opted to add the polyfill to the webpack configuration rather than removing it outright. This way, consumers of the browser files (in dist/) will notice no changes. Consumers of the files in lib/ will either be consuming this package in nodejs or using a frontend build tool to generate output for a browser, where they will presumably already have configured their preferred polyfills (if applicable). The changes here allow such consumers to enjoy a smaller bundle size.