CLI for pg-test and mysql-test

[ForbesLindesay]

Currently we only have the raw API and the jest helpers. I would like us to add:

• start - start a DB and optionally "set" the connection string on an env var somewhere
• stop - stop a DB that was started by the start command.
• with-db - run a given CLI command with a database running and the connection string in an env var

This will become considerably more useful when combined with the work in #23