[feat] Add middleware support as discussed in #293 (#294)

* mux.Router now has a `Use` method that allows you to add middleware to request processing.

middleware.go

@@ -0,0 +1,28 @@
+package mux
+
+import "net/http"
+
+// MiddlewareFunc is a function which receives an http.Handler and returns another http.Handler.
+// Typically, the returned handler is a closure which does something with the http.ResponseWriter and http.Request passed
+// to it, and then calls the handler passed as parameter to the MiddlewareFunc.
+type MiddlewareFunc func(http.Handler) http.Handler
+
+// middleware interface is anything which implements a MiddlewareFunc named Middleware.
+type middleware interface {
+	Middleware(handler http.Handler) http.Handler
+}
+
+// MiddlewareFunc also implements the middleware interface.
+func (mw MiddlewareFunc) Middleware(handler http.Handler) http.Handler {
+	return mw(handler)
+}
+
+// Use appends a MiddlewareFunc to the chain. Middleware can be used to intercept or otherwise modify requests and/or responses, and are executed in the order that they are applied to the Router.
+func (r *Router) Use(mwf MiddlewareFunc) {
+	r.middlewares = append(r.middlewares, mwf)
+}
+
+// useInterface appends a middleware to the chain. Middleware can be used to intercept or otherwise modify requests and/or responses, and are executed in the order that they are applied to the Router.
+func (r *Router) useInterface(mw middleware) {
+	r.middlewares = append(r.middlewares, mw)
+}