Provide local assets directory

[brendan-ward]

This could be very useful for providing local glyphs and sprites, as well as a potentially more generalized solution to local styles than #38

Behavior would be very similar to what we get now with local tiles directory: define it on the command line, and refer to it with a custom hostname, e.g., assets://...

[brendan-ward]

This can also be leveraged for local GeoJSON (intended to be immutable rather than varying with request):
https://docs.mapbox.com/mapbox-gl-js/style-spec/sources/#geojson