Notify.js 4.0 - native ES module

[alexgibson]

This is more a heads up, but when I publish version 4.0 of this library I plan to:

• Publish as a native ES module (all evergreen browsers now support modules natively).
• Remove the transpiled version from the repo (removing both rollup and babel as dependencies).
• Remove Bower support.

If folks want the ES5 compatibility, they can still transpile using their own builds.