Added --no-migration flag to scaffolding

Since the whole point of schema_to_scaffold is to create Rails files around an existing database, a migration should almost invariably be wrong. This changes the default to no-migration, though of course users could still remove the flag manually.

Even after using schema_to_scaffold multiple times, I was still tripping over forgetting to remove the migration. Better not to generate it, since the database will already be in the correct state.