Framework for markdown lint testing

[jayvdb]

Perform markdown linting using three tools,
ignoring many rules to be rectified later.

[jayvdb]

This is a smaller version of #41 , setting up the lint framework only for #36, just adding the linters with extra 'ignore' rules to allow the current text to pass , without any attempt to make the result more parse-able or fix the existing github parsing issues.

[imsky]