Markdown lint in spec tests

[sanfrancrisko]

Originally raised in PDK-958 by @binford2k :

I would love it just for the user experience if the default spec test templates included a markdown lint of README.md and other markdown files. Education uses the mdl tool tool and it's really easy to configure. We could even write a few custom rules to validate Puppet module specific styles – eg, ensure that specific headers exist and have content, ensure that the docs aren't just the example template files, etc...