[WIP] Initial attempt at making map_api_response accept non-json api responses

[gabbifish]

This PR explores a PoC for one part of #26. It focuses on allowing non-json response bodies to be accepted by cloudflare-rs and returned using the same ApiSuccess.result accessor pattern used by preexisting users of cloudflare-rs.

My questionable first approach to this problem: trying to shove the raw text API response into the ApiSuccess.result field, which should ensure that this change is not breaking, but also fails with

error[E0308]: mismatched types
  --> /home/gabbi/github/cloudflare-rs/src/response/mod.rs:39:33
   |
39 |                         result: body, 
   |                                 ^^^^ expected type parameter, found struct `std::string::String`
   |
   = note: expected type `ResultType`
              found type `std::string::String`

^ I'm not sure how to get around this (or if this approach even makes much sense in the first place). I floated other ideas in the comments in the code changes.