serve: Fix corner cases where the pipeline doesn't run correctly. * When `chef serve` is run before the `assets` folder is created, monitor that folder suddenly appearing and rebuild the pipeline. * Do the same when the `config.yml` file has changed.

syntax: glob
*.pyc
*.egg-info
venv
tags
build/lib
util/messages/_cache
util/messages/_counter
dist
docs/_cache
docs/_counter
docs/bower_components
docs/node_modules
piecrust.egg-info
piecrust/__version__.py
.ropeproject