API extension points (#473)

* API endpoint extensions working.

extensions example.

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

extensions example.
example main.go.

* Uncommented special handler stuff.

* Added section in docs for extending API linking to example main.go.

* Commented out special_handler test

* Changed to NewFromEnv

api/const.go

@@ -4,4 +4,8 @@ package api
 const (
 	AppName string = "app_name"
 	Path    string = "path"
+
+	// Short forms for API URLs
+	CApp   string = "app"
+	CRoute string = "route"
 )