Difference.__init__: demote unified_diff to kwarg

Zach Welchrequested to merge
zach.welch.timesys/diffoscope:ztw/cleanup-difference-init into master

This implements most of the changes in the first commit on the WIP/parallel2 branch mentioned in #19. This version preserves the ability to pass the unified_diff at construction, and it drops some unrelated changes that do not belong in this commit.

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