Add optional TLSConfiguration parameter to Postgres' configuration initializer

[seeppp]

A user should be able to call PostgresConfiguration(url:tlsConfiguration:).

In environment like Heroku, where only a DATABASE_URL is given and some databases required SSL on, we should be able to add the TLSConfiguration right on when creating the PostgresConfiguration. Simply adding sslmode=require to the URL does not work as the default TLSConfiguration has TLS verification on.

Related to #186.

[tanner0101]

What about doing something like this instead (which is possible without any changes):

var config = PostgresConfiguration(url: ...)!
config.tlsConfiguration = ...

This usage seems clearer to me especially given that the URL can contain TLS configuration info. In the above example I'm clearly overwriting whatever TLS configuration the url contained with a custom one.

[seeppp]

Okay, that works too. That's why I opened up another PR with the postgress-driver who does what you recommend: vapor/fluent-postgres-driver#153

[tanner0101]

See vapor/fluent-postgres-driver#153 (review)