Output tree-shakeable module

[nsivertsen]

Currently, the build output is a single 32KB UMD bundle. Even if an app only does basic document fetches, it's forced to include everything else in its bundle as well.

Please consider also exporting an ES6 module in the package.module field.

[nsivertsen]

Also, due to the way the global object is set up in the Webpack config you get the following warning when trying to use prismic-javascript as a module using Rollup:

The 'this' keyword is equivalent to 'undefined' at the top level of an ES module, and has been rewritten