Support some form of "reduce" operation.

[nlfiedler]

This library has the "map" concept from CouchDB/PouchDB, but it lacks the corresponding "reduce" that is part of the "map/reduce" paradigm. It is not a critical feature, but it is sometimes useful. In particular, a way to "count" or "sum" the outputs of the map function seems the most common use case.

[nlfiedler]

Added count_by_key() and count_all_keys() which does a query that returns results similar to what a _count CouchDB reducer would produce.