GL() without style on local fs

[frodrigo]

The current GL() function require the style to be on local fs. Could not be a object or a remote ressource:

function GL(uri, callback) {
  this._uri = uri.pathname;
  this._style = require(uri.pathname);

https://github.com/trailbehind/tilelive-gl/blob/master/index.js#L20-L22

The original tilelive-gl has a more flexible construction method.
https://github.com/cutting-room-floor/tilelive-gl/blob/master/index.js#L18-L20