Issues using ES6

@tristen I'm trying to use your mapbox-gl-compare tool using ES6.

Would be really nice if the end result would something like this:

import mapboxgl from 'mapbox-gl'
import Compare from 'mapbox-gl-compare'

// before & after Map objects //

mapboxgl.Compare(before, after)

Uncaught ReferenceError: mapboxgl is not defined | index.js:5

However, the end result looks like this "hack" looking code.

import mapboxgl from 'mapbox-gl'
window.mapboxgl = mapboxgl
import Compare from 'mapbox-gl-compare'

// before & after Map objects //

mapboxgl.Compare = Compare
mapboxgl.Compare(before, after)

Even after doing that, I'm still catching some errors.

Uncaught TypeError: this._setPosition is not a function | index.js:20

Start with the ES6 entry point referenced as index.js and reproduce the reported import usage and errors. Check why mapboxgl is unavailable and why this._setPosition is missing; done means the documented import-based setup works without assigning globals and both reported errors are resolved.