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/runner_async_test.go

@@ -32,7 +32,7 @@ func testRouterAsync(ds models.Datastore, mq models.MessageQueue, rnr *runner.Ru
 	r.Use(gin.Logger())
 
 	r.Use(prepareMiddleware(ctx))
-	s.bindHandlers()
+	s.bindHandlers(ctx)
 	return r
 }