https://svn.python.org/projects/external/Jinja-1.1/docs/build/loaders.html
https://medium.com/@jasonrigden/jinja2-templating-engine-tutorial-4bd31fb4aea3
https://stackoverflow.com/questions/25359898/escape-jinja2-syntax-in-a-jinja2-template
http://click.palletsprojects.com/en/5.x/commands/
https://pymbook.readthedocs.io/en/latest/click.html
tutorial: https://dbader.org/blog/mastering-click-advanced-python-command-line-apps