Feature Request: Envvar specification of the migrations table

[hughobrien]

https://github.com/rubenv/sql-migrate/blob/a32ed26c37ea1bdfd4e08f8aae7b494e7801fb52/sql-migrate/config.go#L82

Datasource is os.envvar expanded, but table is not.

Use case: using sql-migrate for regular migrations (migrations table) but also for gitops sql patches (hotfixes table)