Implement reversal of migration as a command line option

[krafts]

I see the class and code exists for reversing the migration. But the cli option is not exposed so the migration can reversed. A required field would be date, I suppose in this case.

[ccronca]

I think, ff timestamp parameter is set before now, migrations are applied in reverse order.
https://github.com/comeara/pillar/blob/master/src/main/scala/com/chrisomeara/pillar/CassandraMigrator.scala#L48-L52

[comeara]

Yes, that is the case. Thank you for pointing this out. I've left this open as a task for myself to update the documentation to make this clearer.