@configure()

This is a change to config_changes that provides per-class `config` and keeps track of `resourceConstructor` via the new `configure` method, which applies new settings to the current config. Before the same `config` object would be used for every class. 

This allows for:

    class Book extends railsResourceFactory()
      @configure name: 'book', url: '/books'

    class CarManual extends Book
      @configure name: 'car_manual' 

When extending this way, at least one `@configure()` call is required.