Example of database URL and help with special characters

[visnup]

When prompting for the database URL, we should provide an example of the URL format (protocol://username:password@host:port/database). Also, if there are special characters (@:/) in the password, some help on how to escape them properly (@=%40, :=%3A, /=%2F).

[visnup]

Additionally, node-pg supports several flags as URL params which we could document?