Use min_height in building models for bridges and things

[Mapzen](https://github.com/mapzen/vector-datasource/wiki/Mapzen-Vector-Tile-Service#buildings) provides a `min_height` field for parts of buildings that are floating above the ground, like bridges.

You can see some cool examples in NYC at 40.742766592577496, -74.00731522962627 if you zoom in on 10th Avenue and 16th Street.

To use it, you have to make sure to add `min_height` to the `transformation` section of the building output. The basic example is updated accordingly.