Provide a simpler way to use custom extractors

[marcosh]

As far as I understood the code, the way to provide a custom extractor is to register it among the hydrators.

Could it be an idea to allow the extractor parameter in the ResourceMetadata classes to be an object, and not only a string, to allow users to pass directly an instance of ExtractionInterface?

[weierophinney]

Could it be an idea to allow the extractor parameter in the ResourceMetadata classes to be an object, and not only a string, to allow users to pass directly an instance of ExtractionInterface?

No. The idea is that these should be serializable, so that they can be re-used between requests. If we have objects involved, this becomes less manageable and predictable.