Decouple JsonContext from Mink

In order to use JsonContext without mink session (with guzzle for example) we need to let JsonContext fetch the JSON from anything we want.

The idea is to set a listener after each step which listen the result returned by the step.

So for example RestContext returns now the session content : 
https://github.com/tyx/behatch-contexts/commit/1849a3bf94159bb231e05064eacb0cf2ebcb5794#diff-c719e0fc3488be1d7970588ac4102d9cR27

And we inject it in JsonContext because of : 
https://github.com/tyx/behatch-contexts/commit/1849a3bf94159bb231e05064eacb0cf2ebcb5794#diff-b4feec5363c8f57c2d3b10586a532d63R1

So it is not BC Break as I add a very last fallback on mink for now.

(Why?)³

I don't see where the return value is used in this commit. If it needed by other context, add this in another commit.