Add option to set manual rollback as default

[VladislavSokov]

Most likely, users who want to use this task would prefer to have the automatic rollback disabled. Therefore, we should provide instructions on how to disable the automatic rollback. And, if needed, prepare the code for disabling the auto rollback.

For me, the easiest way would be to disable it using an env var, e.g. f ACTUAL_DB_SCHEMA_AUTO_ROLLBACK_DISABLED is defined, then it is disabled. Optionally, they could also disable it via initializer by setting the ActualDbSchema.config[:enabled] flag. But even if it's there, since it's not documented and no info in README, it's hard to know how to disable it, agreed?