Allow style to be passed via URL

[prasadrajuv]

style json is huge in size to pass it has url parameter every time consume high bandwidth. i want to set https://github.com/maputnik/osm-liberty/blob/gh-pages/style.json similar style give url has parameter. mapbox like style default file as argument at time of server start can make avoid style parameter has optional.

[brendan-ward]

Allowing the style to be passed via URL (will be automatically downloaded to the server) instead of in request URL would be a nice addition and would be a nice complement to #51 (could use URLs that resolve to local assets).

Default style is considered in #38 or follow on issues, so keeping that separate from here for now.