Add prerelease promo pricing

In the meantime, have you considered adding a "prerelease" set so we can query those cards as well when pricing things? I know trying to accommodate every possible promo situation probably isn't worthwhile and there was a time when a single prerelease card per set didn't make sense to build a category for but with every rare and mythic in every set now having prerelease versions, there's enough volume that it makes sense.