Supporting additional Charsets

From #72 , @jeremyrsmith raises a question about supporting additional Charsets for Postgres. Currently we are locked in to `StandardCharsets.UTF_8` throughout the code base.

Postgres gives us the current encoding of the Server through a [Parameter Status](https://www.postgresql.org/docs/9.5/static/protocol-message-formats.html) update which can come at any point during the connection.

The question I have going forward - is this something we should automatically detect from Postgres and attempt to make it transparent to a user? Or should we set the encoding through `Stack.Param` configuration and immediately close the connection if the encoding ever changes from the Server?