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/middleware/README.md

@@ -0,0 +1,27 @@
+# Middleware Example
+
+This example adds a simple authentication middleware to IronFunctions. See [main.go](main.go) for example code. 
+
+## Building and Running
+
+```sh
+go build
+./middleware
+```
+
+Then test with:
+
+```sh
+# First, create an app
+fn apps create myapp
+# And test
+curl http://localhost:8080/v1/apps
+```
+
+You should get a 401 error. 
+
+Add an auth header and it should go through successfully:
+
+```sh
+curl -X GET -H "Authorization: Bearer KlaatuBaradaNikto" http://localhost:8080/v1/apps
+```