auto-index [not ready]

**This needs to be rolled out in tandem with a PR to tilelive-bridge:** https://github.com/mapbox/tilelive-bridge/pull/25

TODO:

- tests need to be updated to match the [new fixtures](https://github.com/mapbox/carmen/blob/master/test/fixtures/docs.json) and get everything passing
- verify that tile-cover is working properly around the prime meridian (if not, I think I will update tile-cover to simply split geometries that cross the line, since tiles never do)