Difference.__init__: demote unified_diff to kwarg

diffoscope/comparators/decompile.py

diffoscope/comparators/directory.py

diffoscope/comparators/gif.py

diffoscope/comparators/gnumeric.py

diffoscope/comparators/missing_file.py

diffoscope/comparators/png.py

diffoscope/comparators/text.py

diffoscope/comparators/utils/compare.py

diffoscope/comparators/utils/container.py

diffoscope/comparators/utils/file.py

diffoscope/comparators/xml.py

diffoscope/difference.py

diffoscope/main.py

diffoscope/readers/json.py

tests/comparators/test_binary.py

tests/test_difference.py