Feature Request: Transaction configuration on a per migration basis

## I'm submitting a...

- [ ] Bug report
- [x] Feature request
- [ ] Question

## Current behavior
There is no way to turn off transactions for individual migrations.

## Expected behavior
Certain migrations need to be run outside of a transaction. For example, in PostgreSQL, `ALTER TYPE type_name ADD new_value` cannot be run in a transaction. The rest of the migrations, however should be run in a transaction. There is currently no way to configure individual migrations to not use a transaction block. It would be extremely helpful to allow this to be part of the configuration for a single migration.