Make bodyParser more open and customizable

[DonutEspresso]

Is there value in exposing a way to allow users to pass in a map of types to parsers? Or do we continue to encourage users to layer on their own custom parsers in addition to the currently provided one?

Given that there are some conventions, i.e., req.rawBody and req.body that restify "owns" when it parses a body, there's an argument to be made at least for consistency.