Style as string or struct

[wayeast]

Hello. I am using this crate in a project of my own, and had to make the changes encapsulated here for my own purposes. Just wanted to bundle them into a PR to see if they wouldn't be of interest to you.

Why

The specification for the object passed to the Map constructor in the JS library is documented here (this is the MapOptions struct in this crate). In the JS spec, the style property can be an Object conforming to the Mapbox Style spec or a string that references a document with such a spec. Currently, this crate only accepts the string form of this property. This PR adds the ability for a style spec Object to be passed as well.

What

The core of this PR is the addition of a StyleOrRef enum that expresses the optional Object or string in JS. It also adds the minimal features of a valid Style object (the required properties of the above-linked spec). It also makes some small modifications to existing method definitions to support the new type. Last, it adds a simple example to demonstrate how to use the new enum type.

NB

1. As mentioned above, this PR only adds support for the minimal, required properties of a Style object -- it does not pretend to fully support these objects! If this PR is accepted, it should be followed up with another PR to add all of that support.
2. There are other places in the object passed to a Map constructor that allow for a similar option between a simple type or more elaborate object. This PR is only interested in the style property; if it is of interest to add similar functionality elsewhere, I think this PR may serve as a good pattern to adopt.