Can't toggle DDL Transactions selectively for individual migrations

## I'm submitting a...

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

## Current behavior
At the moment we seem to need to use a `--non-transactional` flag in order to be able to disable DDL transactions from running on a migration. However, it seems that if we only want to do this for a specific migration and not the entire suite, that currently isn't possible.

## Expected behavior
Similar to [Rails ActiveRecord `disable_ddl_transaction!`](https://apidock.com/rails/v5.2.3/ActiveRecord/Migration/disable_ddl_transaction%21/class) it should be possible to disable the DDL transaction from a specific migration without affecting the entire suite

## Environment

db-migrate version: 0.11.3

plugins with versions: `db-migrate-pg` 1.2.2