Add access-callback per public field

#180 

Next is adding tests.
Actually we only care about ALLOW and IGNORED.

Passing the previous response is probably enough for 95% of cases. Every callback is responsible to act only on the predecessor's response. There are some edge cases this will not cover, but I think it's ok.

The alternative would be to stack the responses in an array and pass it along (I'm not a big fan of it, but I can live with it).