Add application-specific metadata associated with migrations

[str4d]

The schemerz table stores a single column with the ID of each migration that has been applied. It would be useful to permit applications to associate metadata with each ID, such as the application's version at the time the migration was applied. This can be useful for debugging, e.g. if two application instances are on A in v1, and one instance migrates A -> B in v2 and B -> C in v3, vs the other instance migrates A -> B -> C in a v1-to-v3 update.