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.

examples/extensions/README.md

@@ -1,12 +1,12 @@
 # Extensions Example
 
-This example adds extra endpoints to the API. 
+This example adds extra endpoints to the API. See [main.go](main.go) for example code. 
 
 ## Building and Running
 
 ```sh
-go build -o functions
-./functions
+go build
+./extensions
 ```
 
 Then test with: