new ResponseFactory implementation

Hi All,

We're using lumen on a set of Micro services and would be helpful to have our own ResponseFactory so we can attach our mappers/transformer for different type of objects, we do know how to do this on current 5.1 branch, but it would be useful if available to everyone in a Lumen way.

1: Create a abstract response factory with responsibility to generate ResponseFactory instances, defaults to current ResponseFactory, and allow attach new ResponseFactories. response() helper would use the Abstract factory with a parameter (prob from env)

https://github.com/laravel/lumen-framework/pull/294/files

2: Change helper to use a ResponseFactory registered in the container and defaulting to current ResponseFactory (not really sure how to implement this, is the container available on helper register?)

Any other suggestions?