Compare with diff-match-patch

* Are there optimizations we should nick?
* Should we provide a comparison in the README as suggested by @josephrocca in https://github.com/kpdecker/jsdiff/issues/225?

Key places to look seem to be:
* https://github.com/google/diff-match-patch/blob/master/javascript/diff_match_patch_uncompressed.js
* https://neil.fraser.name/writing/diff/