Add transit map options

[nvkelso]

See mapzen/eraser-map#710 and mapzen/eraser-map#711 for transit map mode browsing, transit route selection, transit route preview / nav.

[nvkelso]

/cc @bcamper

[nvkelso]

Suggest this can be configured via a new global:

• sdk_transit_overlay which defaults to false but can be set to true. Optionally we might support an auto value which would toggle it depending on variety of circumstances.

There are several different factors that could impact auto including mapzen/eraser-map#710 and mapzen/eraser-map#711. But if the interface should allow someone to force it on or off.

[tallytalwar]

A basic state of false and true seems good to me. I am assuming you will be assigning this global to the visible property of the transit layer(s).

The auto one could be interesting, how do you plan use it in the layers? It might need some more information to determine when to turn a transit layer on/off.

[nvkelso]

This was completed a few months ago.