Procfile - declare commands to be run by Heroku dynosΒΆ

See https://devcenter.heroku.com/articles/procfile. Per http://stackoverflow.com/questions/37080834/can-a-procfile-have-comments, comments are allowed.

Use gunicorn to host the webapp. The options used:

web:
Tell Heroku we’re running a web server, so we can have access to HTTP traffic. Only web process type has this access – see the note just before https://devcenter.heroku.com/articles/procfile#developing-locally.
–chdir build/Flask
Run the application from the Flask directory; see http://docs.gunicorn.org/en/stable/settings.html#chdir.
–log-file -
Report log output to stdout? The manual’s not very helpful. See –log-file -.
–error-logfile -
Report errors to stderr? Again, http://docs.gunicorn.org/en/stable/settings.html#logging isn’t terribly clear. Without this option, Flask tracebacks don’t appear.
book_webapp:app
Per http://docs.gunicorn.org/en/stable/run.html#gunicorn, the WSGI callable for the app is book_webapp.app.
web: gunicorn --chdir build/Flask --log-file - --error-logfile - book_webapp:app