PostgreSQL dialog URL parser

[tombiscan]

After some improvements from the #281 proposal have been implemented by @sio4 some nice options to improve the PG dialog have been opened.

lib/pg by default converts any provided URL type of string into specific connection string.

gobuffalo/pop by default parses the URL for PG. In practice, that means any attempt to use connection format string will fail due to the URL parsing something that's not a URL.
Notably, that also affects Google Cloud SQL, which uses the mentioned string format to connect.
Rationale and previous "quick fix" can be seen here: #281 (comment)

This PR:

• implements URL parser for PostgreSQL by replicating the behaviour from lib/pg (and shamelessly copying few parser funcs from there)
• the parser now sets all the additional options to ConnectionDetails.Options
• adds some tests
• "works for me" on GCP with App Engine Standard and Cloud SQL
• helps with making Gobuffalo GAE compatible - Make Buffalo GAE compatible  buffalo#213

[stanislas-m]

LGTM. :)