Add a maxDepth parameter

[jlivak]

This library was just what we were looking for, except we found on very large, very deep objects performance would slow to a crawl because it splits up every single object or line.

Adding an optional maxDepth allowed us to increase performance by grouping deeper objects into a single stringify, and we found that it decreased the time it took to fully stringify one of our test objects (~500MBs of data) from ~90 minutes to ~20 seconds, while still avoiding out of memory issues traditional JSON.stringify() had.

[kibertoad]

@jlivak Can you address the conflicts?