Have an option of disabling the automatic transaction in migrations

[dantownsend]

When migrations are run, each one is automatically wrapped in a transaction.

This is useful behaviour the majority of the time, because it means the changes are rolled back if the migration fails half way through.

However, if writing a custom migration, which might involve complex logic, having direct control over the transaction can be beneficial.

We should add an option like wrap_in_transaction to MigrationManager, which defaults to True.