CI for documentation: Testing if documentation/tutorial is runable.

To test a tutorial, you can write docable steps:
setup:
# local: {}
slim:
image: ubuntu16.04-php
unix-service.md:
steps:
- selectAsFile: ROT13 transformation => server.php
- selectAndServe: "start it:"
- selectAndExpect: "another terminal:"
Then execute these tests to generate docable report:
docable report steps.yml
- Related publications: Evaluating the Executability of Software Tutorials