Add sslmode parameter to postgresql uri and dsn
I think it would be good to add the sslmode parameter to the PostgreSQL backend connection uri and to the database dsn method [0]
Services like Digital Ocean Managed Databases [1] requires encrypted connection to the database and need the sslmode set in the dsn.
[0] http://initd.org/psycopg/docs/extensions.html?highlight=sslmode#psycopg2.extensions.ConnectionInfo.dsn_parameters
[1] https://www.digitalocean.com/products/managed-databases/