Support external credentials files/env vars

[danielkza]

Would make it much easier to automate migrations in different environments.

[fernandezjose]

Doesn't #23 PR partially solve this issue?

[danielkza]

@ppcuban It does help for people that rely solely on TLS for client authentication, but when using password authentication (either in addition, or on it's own) a credential file would still be helpful.

What I would like to avoid is for people to include secrets in the same repository as their migrations, as I find that to be much more prone to human errors, such as accidentally working in a production database when you didn't mean to.

[Roguelazer]

Would you be open to a commit that adds a --password-environment-variable-name option which would read the password from an environment variable so that it (a) is easier to automate in multiple environments, and (b) doesn't leak passwords in ps?