Unable load sprites from file:/// schema.

[oscarfonts]

mapbox-gl-js version: 0.36.0 (and master at the time of writing)

Steps to Trigger Behavior

1. Configure a style with a sprite url starting with "file:///"
2. Instantiate a Map

Expected Behavior

Load the sprites and use them.

Actual Behavior

Sprites are not shown. Error thrown:

 Error: Unable to parse URL object
      at parseUrl (src/util/mapbox.js:104:15)
      at Object.exports.normalizeSpriteURL (src/util/mapbox.js:64:23)

Origin

The origin of the error is a regular expression urlRe defined in src/util/mapbox.js, which doesn't match file:///path:

/^(\w+):\/\/([^/?]+)(\/[^?]+)?\??(.+)?/

It expects the "host" part of the URL (2nd capture group) to have some value, but in fact it can be empty (as in this case).

Fixture

Making the 2nd capture group optional would fix the issue.
Will provide a PR with the test case and a patch.