Add way to set graphql.Params.RootObject via handler.Config

[nossila]

I wanted to have http.ResponseWriter and http.Request within my relay.MutationConfig .MutateAndGetPayload and graphql.Field.Resolve methods. The main reason is that I needed to write and read cookies for authentication reasons.

Looking on #1 and #7 seems like there are other cases where this PR would be helpful. In my case with this changes I just needed to write a very single http handler to have access to both http.ResponseWriter and http.Request anywhere:

func requestResponseOnRootObject(h *handler.Handler) func(http.ResponseWriter, *http.Request) {
    return func(w http.ResponseWriter, r *http.Request) {
        h.RootObject = map[string]interface{}{
            "w": w,
            "r": r,
        }
        h.ServeHTTP(w, r)
    }
}

graphqlHandler := handler.New(&handler.Config{
    Schema: schema.GetSchema(),
    Pretty: true,
})
http.Handle("/graphql", requestResponseOnRootObject(graphqlHandler))

[coveralls]

Coverage increased (+0.1%) to 94.545% when pulling d86c8a0 on nossila:master into 52e6de4 on graphql-go:master.

[chris-ramon]

Thanks a lot for your contribution @nossila 🌟 👍 — closing this one in favor of: #30