serve: Add server sent events for showing pipeline errors in the debug window. The server can now run an endpoint that streams pipeline status, including errors or "fixed" statuses. As a result, I had to investigate using event-loop based server alternatives, before I figured out the correct flag to set in Werkzeug. Support for Gunicorn is therefore now possible, although disabled by default. I will come in handy though when proper support for CMS-mode is enabled.

*.pyc
venv/
build/lib
build/messages/_cache
dist/
piecrust.egg-info/
piecrust/__version__.py