Process columns ending in "__json" as JSON objects, not strings

This lets you store JSON strings in your database and have Elasticsearch index them as objects.

The code is very rough/hacky and does not include config settings or docs, but I wanted to submit it to see if you had any suggestions/ideas. I'm willing to clean it up and resubmit it if you think it would be useful.

Motivation: We use the PostgreSQL JSON support in PLv8, which means we store JSON strings in our DB. This also would help people who use FriendFeed-style MySQL JSON schemas.