Migrate development data

I'm finding it more and more useful to have some pre-seeded fake data in the database during development, but this sort of data can't end up in a migration.

Would it be a good idea to have a query/_migration/dev directory to apply AFTER the migration completes for the first time (or with --force), that's only applied on development - maybe with a --dev flag?

Complete #357 first, then having dev-001.sql in the migrations directory won't break anything.