add allow_missing_migration_files as option for migrations

We provide a web server that is deployed to client machines in a package, and each server has its own local database. We auto-migrate each database on startup of the web server to facilitate simple upgrades (the client just has to start the web server). There are cases where we want to be able to allow a client to use an older version of the web server, but asking them to understand how to migrate their database is cumbersome. We take care to not break older code with migrations, so they should be able to happily run older versions on newer databases, except that Sequel complains. I've added a migration flag to potentially stop complaining if the state of the DB and the migration folder is not a perfect match.