feat: character diffs

[KnorpelSenf]

Builds on top of the changes in #33 in order to add support for character diffs. Should be reviewed only after #33 is merged. The diff is here: https://github.com/KnorpelSenf/imara-diff/compare/word-diff...KnorpelSenf:imara-diff:char-diff

While a character diff can technically be performed directly on a line diff, this implementation supports performing a character diff on top of a word diff, too. That way, one can:

1. perform a line diff
2. for each line hunk, perform a word diff
3. for each word hunk, perform a char diff