add Mapbox geocoder

This adds support for the [Mapbox geocoder](https://www.mapbox.com/developers/api/geocoding/), both forward and reverse. Mapbox powers Foursquare, Pinterest, Evernote, GitHub, Square, and others. 
Line is too long. [90/80]<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.