Support other map providers like MapTiler

[almccon]

This seems distinct from #35, but if we want to combine these issues, that's fine with me.

MapTiler works very much the same as Mapbox, you would specify an access token and a style URL.

Of course, it already seems to work currently if you include your key in the URL, like so:

  {
    id: 'maptiler-toner',
    name: 'MapTiler Toner',
    type: 'mapbox-gl',
    url: 'https://api.maptiler.com/maps/toner/style.json?key=YOURKEYHERE',
  },

...so maybe we don't need to do anything other than add this to the documentation to show how it's done?