Add functionality to use Postgres env vars for configuring cigogne

[enderdwt]

It would be nice to add the ability to configure the database connection with the Postgres env variables like squirrel does here. This would allow me to use the same env vars for my postgres docker image, squirrel, and cigogne. If this is something that you are open to, I would be happy to create a PR for this change.

[Billuc]

Closed by #48