Fix: Implementing Character-Level Diffing in Limited Situations

[houfu]

Closes #44

We make a limited two pass diffing approach which replace words that share similarities by running a second character level diff on just those pairs (like "dinner"/"dinners"). This is only activated when the change is only on one word/token has changes in the beginning or end of the token. This is a balance on performance and improvements on common cases. You might find inconsistencies or puzzling cases why character level is or is not activated. Feedback is always welcome.