Build out /styles endpoint

[chriswhong]

POST /styles

The styles endpoint returns a valid mapboxGL style based on the base style, modified using the following parameters:

excludes (Array) - Array of layer ids from the base style to exclude.

overrides (Array) - Array of mapboxGL layer fragment objects including the layer id. Each key in this object will override the default in the corresponding layer from the base style.

sources (Array) - Array of mapboxGL source objects to be included in the returned style. Accepts all existing mapboxGL source types plus the ‘cartovector’ type defined in this spec.