Diff support for mendoza-js

[djherbis]

It's great that you have patch support, it would be awesome if diff was also supported in the frontend.

Use case: differential saves. You have a large JSON document in your frontend and want to send diffs instead of the whole document to your backend to update the saved copy.

[judofyr]

That would indeed be very cool, however the current diff algorithm used in the Go implementation is quite complicated and we don't have any immediate plans to re-implement it in JavaScript.

We'll happily merge a diff implementation if someone else wants to contribute though!