[WIP] Add array mapping value transformer

This transformer applies the given transformer to each element of an array. I've found this useful where my JSON contains arrays of strings (e.g. URLs) and other non-dictionary values that can't be represented by an `MTLModel`.

At present this requires a reversible transformer. I'm not sure whether one method (`mtl_arrayMappingTransformerWithTransformer:`) can handle both reversible and one-way transformers, but a second method could be added for this case.