Add sample configuration loaders for inMemory and ciab

[jeffmcaffer]

Sketched out implementations of two different config loaders that go with the PR from @patrick-steele-idem for refactoring the crawler factory. That PR should end up in this branch eventually.

This change takes a poor man's dependency injection approach (thanks @willbar). The basic idea is that a config loader creates effectively a dependency injection container full of services that make up the crawler (including the crawler itself). Each service is created by a factory (or whatever you want). The standard pattern is for the factory function to get a container of services previously created, an optional name (if provided it should add itself to the container) and an optional set of default settings.

There is also a philosophical change in that the provider code should have core defaults built in so the user does not have to spec any/many options. We'll have to document the available options for each provider but that's a good thing anyway.