allow raw Postgres connection string

Allow Postgres connection settings via params or a raw connection string. The latter is especially useful when connecting to remote Postgres databases, since Postgres allows connection settings to be provided via params or a URI.

Specific use case: hosted database services like Heroku provide users a full connection URL. Connecting to this via Hasql currently requires deconstructing this URL into its various parameters. Allowing users to simply pass the connection string is hella convenient. :wink: 

Depends on PR https://github.com/nikita-volkov/hasql-postgres/pull/7.