Add an API for getting the JSON diff as an object instead of a string

[zbanks]

Motivated by decomp.me.

In #52 I added the JsonFormatter for returning the diff results as a JSON blob. This is fine for CLI use, but decomp.me imports diff.py directly and uses it as a library. If we're worried about diff performance in decomp.me, diff.py could expose a function for getting the diff result as a dict, rather than as a serialized str.