Add an option for user-defined middleware after the built-in middlware, to modify the RawRequest object #62
Description
I wanted to use resty for requests to an AWS endpoint that requires requests to be signed with AWS credentials. A library already exists for this (https://github.com/smartystreets/go-aws-auth), however using that one requires access to the http.Request object directly. Currently, user defined middleware is run before the built-in middleware, which makes sense when modifying the resty.Request object. However for resty to be compatible with go-aws-auth, I need middleware that is processed after the built-in middleware, so that the RawRequest object is available.