Add route costing model global to scene file

[nvkelso]

Paired with a new application_state Tangram scene file global that Eraser Map would set (proposed in https://github.com/mapzen/eraser-map/issues/710), let's also add a new global property for Route costing model:

ux_route_costing_model

• auto (x)
• bicycle (x)
• multimodal (x)
• pedestrian (x)
• auto_shorter
• bus

(x) supported in Eraser Map

See also:

• https://mapzen.com/documentation/turn-by-turn/api-reference/#costing-models

[nvkelso]

Related: map style switcher in https://github.com/mapzen/eraser-map/issues/700.

[sarahsnow1]

Closing in favor of https://github.com/mapzen/eraser-map/issues/820