Let DATABASE_URL configure any database.

This change introduces a parser for the `DATABASE_URL` environment variable early in the configuration stage, so it can be used to configure any database using any driver.

`db-migrate` already supports using `DATABASE_URL` in the Postgres driver. This change makes the support generic.

I hope that you will consider merging it! I'll be happy to address any feedback about the code structure or style.

Thank you very much for `db-migrate`!