Add Connect-like middleware

[jergason]

It would be awesome to have some kind of middleware to pass the request and response to once the request finishes.

Maybe pass in an array of middleware functions when you require ahr, and then pass the results of NativeHttpClient(req, res) through each of the middlewares. I am still trying to wrap my head around the code a little bit though.
@Gissues:{"order":20,"status":"backlog"}