Migration Table add "sql" VARCHAR col (traceability)

So currently the `migrations` table has `id`, `name`, and `run_on` as columns. I am proposing to either add a `sql` column to actually save off what has run for traceability.

Granted this is pumped to stdout and could just be utilized in your CI architecture.

Even if this is not accepted would it be possible to make `createMigrationsTable`'s options more flexible so that custom columns could be added more easily.

Seems hardcoded [here](https://github.com/db-migrate/pg/blob/master/index.js#L163-L175)