Read API base URL from Info.plist

If no host is passed into Geocoder(accessToken:host:), the fallback hostname should be either api.mapbox.com or api.mapbox.cn depending on the value of MGLMapboxAPIBaseURL in Info.plist:

https://github.com/mapbox/MapboxGeocoder.swift/blob/faa288dcf09f9dbcab1a54e61e741932f3945239/MapboxGeocoder/MBGeocoder.swift#L154

This would be consistent with how we read the access token out of Info.plist:

https://github.com/mapbox/MapboxGeocoder.swift/blob/faa288dcf09f9dbcab1a54e61e741932f3945239/MapboxGeocoder/MBGeocoder.swift#L147

This will make it easier for developers to keep their map and geocoder usage in sync, since Geocoder.shared would automatically pick up the API endpoint specified in Info.plist. Developers who need a different value for the geocoder can still override the API base URL when creating a separate Geocoder instance.

/cc @pdgoodman

从 MapboxGeocoder/MBGeocoder.swift 第 147-154 行附近开始，将 Info.plist 中现有的 access-token 查找与回退 host 逻辑进行比较。当省略 host 时从 MGLMapboxAPIBaseURL 选择 api.mapbox.com 或 api.mapbox.cn，而显式提供的 host 仍然作为覆盖值时，修改就完成了。