Add support for complex properties from GeoJSON and TopoJSON formats

[nvkelso]

Followup based on root cause of #1616.

Tilezen tiles (docs) include nested properties on several properties, including the following. While these work in Tangram JS, it seems like Tangram ES doesn't fully support this today (and should) as these are valid file format options.

roads layer lists:

• all_networks
• all_shield_texts
• all_walking_networks
• all_walking_shield_texts
• all_bicycle_networks
• all_bicycle_shield_texts
• all_bus_networks
• all_bus_shield_texts

Paired with some single props &/or booleans to make life easier:

• network and shield_text
• walking_network and walking_shield_text
• bicycle_network and bicycle_shield_text
• bus_network and bus_shield_text
• is_bicycle_related
• is_bus_route

pois layer lists:

Each station has several (optional) lists of routes per transport type:

• train_routes a list of train routes, generally above-ground and commuter or inter-city “heavy” rail.
• subway_routes a list of subway or underground routes, generally underground commuter rail.
• light_rail_routes a list of light rail or rapid-transit passenger train routes.
• tram_routes a list of tram routes.

Paired with some booleans:

• is_* a set of boolean flags indicating whether this station has any routes of the given type. These are, corresponding to the above *_routes. This is provided as a convenience for styling:
  • is_train
  • is_subway
  • is_light_rail
  • is_tram

[nvkelso]

MVT support is different (the data would come stringified instead of in object notation) so I've split that off into: #1618.