Incorporate WOF lookups for region boundaries

[louh]

See various demos:

Forward geocoding with boundaries: https://mapzen.github.io/leaflet-geocoder/examples/show-boundaries.html

Reverse geocoding with boundaries: https://mapzen.github.io/demo-search-and-wof/reverse-demo.html (code)

This has been neither the priority of Pelias / Search nor the WOF teams because of it involves tying together two separate projects, but that's why it makes sense to do it within mapzen.js.