Improve choices in rake setup

[pjungwir]

This PR bundles three minor improvements, all around Postgres connection parameters:

• You can give a postgres port, in case you have multiple versions of postgres installed.
• We urlencode the password in your connection URL, in case it has special characters.
• We print the correct default database name (postgres_ext_serializers_test vs postgres_ext_test).

Btw I'd be happy to take over maintainership of this gem if you have abandoned it. (I realize I probably need to renovate postgres_ext too or switch this gem to ctes_in_my_pg.) I've taken the approach of generating JSON in Postgres on a bunch of projects, but with manual SQL, and having this gem would be a huge improvement. Let me know what you think!