Add a watch mode to the generate command (automatic, continuous rebuild)
The generate command should provide a watch mode. In this mode, the site generator will monitor files on the local filesystem for changes. If a change is detected, the generator pipeline should run again.
Ideally, the generator will only process files that changed. However, this can be tricky as a change to a partial file won't produce a meaningful result. Therefore, perhaps all local files should be processed again.