Make it easy to implement a document-based configuration source that falls back on the main wiki

Currently it's easy to implement a document-based configuration source component that looks in the current wiki: simply extend AbstractDocumentConfigurationSource and implement 3 methods to return the configuration document reference, class reference and cache id.

If you want to fall-back on the main wiki then currently you need to implement 2 configuration sources: one that looks in the current wiki and one that looks in the main wiki. Moreover, you need to implement the fall-back yourself, inside your configuration component.