Add Match(..) bool to chi.Routes, and add middleware.GetHead to automatically route missing head requests to GET handlers

* Put chi.Routes pointer on the route Context instead of having to pass to the middleware

* Rename FindHandler to Find, and add breaking test when searching subtrees

* Rename middleware to GetHead, and update Mux#Find method to search subroutes

* Update README, router at ~1000 LOC

* Redefine Routes#Find(..) http.Handler to Routes#Match(..) bool

* Update benchmarks

* Slightly adjust GetHead middleware to make it clearer