Allow configuring Postgres password through the socket URL.

For example:
```ruby
  ActiveRecord::DatabaseConfigurations::UrlConfig.new(
    :production, :production, 'postgres:///?user=user&password=secret&dbname=app', {}
  ).configuration_hash
```

will now return,

```ruby
  { :user=>"user", :password=>"secret", :dbname=>"app", :adapter=>"postgresql" }
```