Adding in an exported method that allows clients to retrieve a full list of Handlers for the current Negroni chain.

Hello there!

First off, thanks for the great library :smile:!  It's been very helpful in providing much needed diagnostics for our internal golang services and for some of my experimental projects as well.

Lately, I've been wishing that the library had a way to inspect the full list of Handlers so that I might be able to create more flexible middleware.  For example, this might be useful when you want to reflect on your final `http.Handler` in order to efficiently handle Content-Type Encoding negotiation, Request Body mismatches, or provide other interesting diagnostic information.

This PR is intended to provide clients of `negroni` this ability by returning a list of all the Handlers for any given Negroni middleware chain for inspection.

Let me know what you think, and thanks again!