Iterator version?

[LiraNuna]

This library is awesome, but one thing it suffers from is huge memory consumption on large strings as the resulting array can be pretty gigantic. I would love a generator version that spits out small chunks instead of a giant result array.

I attempted this by myself but unicode and "cleanup" turned out to be out of my understanding.

See https://github.com/jhchen/fast-diff/compare/master...LiraNuna:fast-diff-iterator:master for my attempt.