Support JOINs in Mango

@davisp:
> JOINs make me squeamish.

@janl:
> I hear ya, but devs love em :) — And a native API would simplify a lot of app and library code.
>
> I’d definitely say it shouldn’t be full SQL JOIN semantics, but a simple LEFT JOIN on a key over two views should be fine and address a good bunch of use-cases. Basically, anything that isn’t conceptually different from merging multiple shard-view results on the top node, just with two views.

@nolanlawson:
> Relevant pouchdb-find discussion: https://github.com/nolanlawson/pouchdb-find/issues/157

Start by reading the Mango implementation and the linked pouchdb-find discussion to understand the proposed JOIN scope and existing constraints. This issue is ready only once the JOIN behavior and API are defined, implemented, and covered by appropriate tests; the payload names no specific files or test entry points.

Written by the indexing model from the issue text.