`default_configuration` does not propagate along fallback chain like `config_methods`

I'm using Dragonfly to write a general web scraping framework (not Rails! not images!). I perform introspection on `config_methods` and make them available as command-line options to the scraper. [See here](https://github.com/opennorth/unbreakable/blob/master/lib/unbreakable/scraper.rb#L181)

However, I've noticed that if I have `configurable_attr` on a custom processor, its `default_configuration` does not propagate up to the app's `@processor` (or to the app). This is because `use_as_fallback_config` doesn't merge `default_configuration` from the child configurable.

I added a test to illustrate the problem.