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/models/datastore.go

@@ -6,6 +6,7 @@ import (
 )
 
 type Datastore interface {
+	// GetApp returns the app called appName or nil if it doesn't exist
 	GetApp(ctx context.Context, appName string) (*App, error)
 	GetApps(ctx context.Context, filter *AppFilter) ([]*App, error)
 	InsertApp(ctx context.Context, app *App) (*App, error)