Support int8 migration versions via new int8-versions feature

[mpalmer]

This addresses, though does not really fix #83, because it doesn't make refinery support timestamped migrations by default, but only if you opt-in to the new feature. However, making it an optional feature neatly sidesteps the unanswered questions in the issue, and so makes the implementation easier to complete and land.

[mpalmer]

Would you be willing to do that instead?

Not at the present time, no. As you identified in #83, there are unanswered design questions with migrating existing users to an int8 column, I don't use the CLI, and the approach in this PR Works For Me(TM). Perhaps one of the commenters from #83, such as @ankhers, might like to use this PR as a basis?