Add option for a dry run

Some other migration frameworks such as node-db-migrate offer a --dry-run option. This simply prints out all the SQL statements that would be run, but does not actually touch the database.

This can be a very useful check prior to running a migration on a database with real data. It also can be a bit of a sanity checker, unlike normal database queries, migrations are difficult to write tests for. So this would also allow developers to actually verify that their migration file runs, without running it.

I'd be happy to work on a PR with some direction on where to get started, would this involve changes in Sequelize?