add features from blevesearch/geo fork

[jmr]

There's a fork at https://github.com/blevesearch/geo with some missing functionality like region term indexer.

It would be great to get those merged here.

@sreekanth-cb @abhinavdangeti @iulian-gm

[panmari]

@abhinavdangeti I saw that you recently merged changes from this repo to your fork. Is there interest to merge back some of the missing functionality you implemented and we also have in C++, e.g. S2RegionTermIndexer?