Feature: improved and configurable threshold for text similarity

[zmbc]

Currently, it appears that if the source of two cells is not at least 70% the same, they will always be treated as separate cells and represented by a delete-cell, then add-cell operation. It feels like this threshold should be much lower, at least if whitespace-only lines are ignored.

There appears to be a relevant TODO:

nbdime/nbdime/diffing/generic.py

Lines 30 to 31 in 458beeb

	# TODO: Add configuration framework
	# TODO: Tune threshold with realistic sources