Allow role and database configuration

`FATAL:  role "demo" does not exist` happens a lot when using this in test suites, which is related to this line:

https://github.com/CQCL/tmp-postgrust/blob/5208745c474a98f9eb50d7ac1c18a671618f830d/src/lib.rs#L200

The nearby TODO suggests this ought to be configurable, though I don't remember exactly what my intention was there.