Let the client configure the JSON library

[GPrimola]

After Phoenix opinionated on Jason as its default JSON library (configurable), there's a tendency to generally use Jason.
Regarding white-bread, I think the best approach would be to be non-opinionated regarding that and let the client configure that, since Jason and Poison have the same interface (behaviour) it wouldn't be a problem.
For the client to use I suggest something like:

config :white_bread, :outputters
  json_library: Jason

Thank you!