add go.mod, assuming current master is a v3 branch

Import paths are changed according to Semantic Import Versions rationale.
Otherwise `vgo test ./middleware` fails bacause of import path interpretation - it assumes
that "github.com/go-chi/chi" points to v1, fails to find some public functions from v3 and dies.

As semantic import versioning seems to be the official way, we should respect it.

This is an experimental way of implementing go-chi#302 without major version bump.
Backwards compatibility is done via `v3` symlink - old go is OK with it.

Release v3.3.1

Release v3.3.0

Release v3.2.1

FindRoute now returns specific method handler along with endpoints map

go vet and golint the project, release 3.1.5

Ensure enough param values in mount handler

Implement router tree Walker, an analogy to filepath.Walk (go-chi#222)

* Implement router tree Walker, an analogy to filepath.Walk

func Walk(r Routes, walkFn WalkFunc) error
type WalkFunc func(method string, route string, handler http.Handler, middlewares ...func(http.Handler) http.Handler) error

* Move Walker to tree.go

Flatten Context.URLParams data structure