Add SSLMode connection option

[IAkumaI]

PR: #75

For some databases I need to control SSL mode option (actually, need to disabled it), otherwise it does not connect.
So I have added the option to CLI config.

[tianzhou]

A properly configured PostgreSQL server should handle prefer gracefully. You shouldn't need to disable SSL unless PG server SSL is broken somehow.

[IAkumaI]

Yeah, but sometimes it's just so. Might be it's a dev server or what ever.
Just setting the option to disable helps, otherwise I get FATAL: password authentication failed for user \"postgres\" (SQLSTATE 28P01) error.

Sometimes it's easier to pass a right config rather than changing the server.

[tianzhou]

No obvious use case