Detect all fields of objects in JSON array for filtering.

This PR adds logic to detect all fields of all of the objects in a given JSON array for filtering. When benchmarked with https://gist.github.com/zachlatta/f325dacf1866b50a01b1 it, on an average calculated from 1000 iterations, took 0.805 milliseconds on my MacBook Air with an i7.

Closes #7.