Reverse Geocoding with CesiumJS?

It _seems_ like it should be possible to reverse geocode with CesiumJS, but I don't see any functions exposed for this in the docs. This [blog post](https://cesium.com/blog/2018/08/14/geocoding-with-pelias/) describes how Pelias's geocoder supports reverse geocoding but doesn't talk about how to do it.

We should expose it and/or explain how to use it. This came up in a forum thread here: https://groups.google.com/d/msg/cesium-dev/5CTEv5D7LGI/bYGYh9Z1AQAJ

Start by reviewing the CesiumJS geocoder documentation and the linked Pelias reverse-geocoding blog post, then compare them with the forum discussion. Done means either exposing a reverse-geocoding entry point in CesiumJS or documenting the supported way to use Pelias for it.

Written by the indexing model from the issue text.