Support SSL/TLS for postgres_exporter <-> PostgreSQL

1. It should be possible for postgres_exporter to connect to PostgreSQL with SSL/TLS. There should be options in DSN for:
   1. enabling/disabling SSL/TLS;
   2. enabling/disabling insecure TLS (InsecureSkipVerify).
2. If there are no options for that yet, they should be added in a way that is similar to other existing options.
3. There should be an automated (on Travis CI) test for that.

See Smart Checklist.