Styling

Currently, only the `color` property is supported.
Could be extended to something similar to http://geojson.io/ :

- Point
  - `marker-color`
  - `marker-size`
  - [`marker-symbol`]
- Line
  - `stroke` (color)
  - `stroke-width`
  - `stroke-opacity`
- Polygon
  - `stroke` (color)
  - `stroke-width`
  - `stroke-opacity`
  - `fill` (color)
  - `fill-opacity`