Add a config option for setting allowed sets of duplicates for W002

[jwodder]

The config option should take a sequence of sets of file paths. If two files in the same set are duplicates of each other, W002 should not be raised (but if a file in a set is duplicate of a file not in the set, the check should still fail).

[henryiii]

This could also default to ignoring __init__.py, as well? That's often correctly an identical file. Ahh, I see #2. Thought I think when the config option is added __init__.py should probably be the default setting. :)