v3.0.0

• major upgrade of whatwg-fetch to 3.0.0 (see breaking changes)
• minor upgrade of node-fetch to 2.3.0 (see changelog)
• comprehensive test coverage (from 16 specs to 72)
• dropped yarn in favor of npm for package management
• dropped npm in favor of make for tasks runner
• Now you can do import {fetch} from 'cross-fetch' on a browser bundle.
• For ponyfills, fetch.polyfill is now set to undefined instead of false.