Read API base URL from Info.plist

If no `host` is passed into `SpeechSynthesizer(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/mapbox-speech-swift/blob/b0163852eb6e3efd36effc9893e1eab10932036a/MapboxSpeech/MapboxSpeech.swift#L96

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

https://github.com/mapbox/mapbox-speech-swift/blob/b0163852eb6e3efd36effc9893e1eab10932036a/MapboxSpeech/MapboxSpeech.swift#L89

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

/ref mapbox/MapboxGeocoder.swift#107
/cc @m-stephen @bsudekum