read `url` parameter for GeoJSON sources

[jfirebaugh]

GeoJSON sources should probably use the url parameter for a URL, like other sources, and reserve data for actual inline GeoJSON. See #758.

[tmcw]

Pretty confused at this point - if you're declaring a GeoJSON source in a stylesheet rather than in JS as a new GeoJSONSource, how do you specify data other than as a url?

[jfirebaugh]

You can say:

"geojson": {
  "type": "geojson",
  "data": {
    // GeoJSON goes here
  }
}

or:

"geojson": {
  "type": "geojson",
  "data": "http://geojsonurl"
}

I think we should replace support for the latter with support for:

"geojson": {
  "type": "geojson",
  "url": "http://geojsonurl"
}

[jfirebaugh]

Ticketed in style-spec.