Get geojson from url if one is specified

I noticed that `GeoJSONSource` only works if you provide raw data to it as `data` param. I really like declarative approach of `style.json` files and thought it would be great if one could specify GeoJSON datasources as urls to geojson files and not having to do the fetching in application code.

Here's a basic example I've been using: 

``` javascript
mapboxgl.util.getJSON('http://localhost/lab/style.json', function (err, style) {
  var map = new mapboxgl.Map({
    container: 'map',
    style: style,
    center: [58.5793159, 16.190499],
    zoom: 10
  });
});
```

And here's the style file:

``` javascript
{
  "sources": {
    "parks": {
      "type": "geojson",
      "url": "data/parks.geojson"
    }
  },
  "layers": [
    {
      "id": "parks",
      "source": "parks",
      "style": { "fill-color": "#98E97F" },
      "type": "fill"
    }
  ]
}
```