tvOS support

[djensenius]

tvOS does not have MapKit, and would love to have MapBox as an alternative.

[friedbunny]

tvOS seems rather fun and Mapbox does run on it, but time and device limitations may prevent us from doing much more than non-interactive maps (in the short-to-midterm).

Interested parties should see @webventil's fork (https://github.com/webventil/mapbox-gl-native/commits/tvOS), which adds a build target. There's been some implementation discussion in #2340, as well.