Add unique validation rule

[bsweger]

Noticed that this is set as "todo" in the validator.py 😃

Looks like we'll also need to fix up the unique column of the rules file to accomplish this. I think we should just assume that all fields labeled as unique = True in the rules file represent composite unique keys for the file and should be checked for uniqueness as a group (i.e., file-level validation rather than row-level).