Middleware (#502)

* API endpoint extensions working.

extensions example.

extensions example.

* Added server.NewEnv and some docs for the API extensions example.

extensions example.

extensions example.

* Uncommented special handler stuff.

* First example of middleware.

easier to use.

* Added a special Middleware context to make middleware easier to use.

* Fix tests.

* Cleanup based on PR comments.

api/server/special_handler_test.go

@@ -10,7 +10,7 @@ func (h *testSpecialHandler) Handle(c HandlerContext) error {
 }
 
 func TestSpecialHandlerSet(t *testing.T) {
-	// temporarily commented until we figure out if we want this anymore
+	// todo: temporarily commented as we may remove special handlers
 	// ctx := context.Background()
 
 	// tasks := make(chan task.Request)