Support pgx as a postgres driver

[crazybolillo]

I mostly use pgx in my projects, however to run migrations as a library with dbmate, I must use the postgres driver package (as far as I understand) which uses libpq. This means I have to pull two different SQL drivers.

Is it possible to use pgx instead? Or maybe add a new package postgres-pgx which pulls in pgx instead of libqp?

[crazybolillo]

Also, if this is accepted, I am willing to work on a PR for it.