New init method that wraps new Map()

[stevage]

This might give a convenient way to alter the style before the map is loaded.

I'm thinking something like:

const map = U.create(mapboxgl, {
  container: 'map',
  style: 'mapbox://styles/blah.foo',
}, {
  transformStyle: style => {
...
});

[stevage]

It looks like I started implementing this (there's a function called newMap() which behaves a bit like this) but I have no memory of it.