aline difference marking is done on the
similar pair. Lots of work, but often worth it.

Example:

>>> d = Differ()
>>> results = d._fancy_replace(['abcDefghiJkl\n'], 0, 1,
...                            ['abcdefGhijkl\n'], 0, 1)
>>> print(''.join(results), end="")
- abcDefghiJkl
?    ^  ^  ^
+ abcdefGhijkl
?    ^  ^  ^
g