Register (and handle) tilejson+(https?|file): protocols

This allows callers to distinguish between tile sources w/o available TileJSON endpoints (e.g. `http://tileserver/{z}/{x}/{y}.png`, using [`tilelive-http`](https://github.com/mojodna/tilelive-http)) and HTTP-accessible TileJSON (e.g. `tilejson+http://tileserver/index.json`, using `tilejson`).