Move to async/await

[BlindChickens]

If the current lib/htmllint.js code can be rewritten to make use of async/await it will be more readable and maintainable.

The tests would then also be more readable and more versatile. I could get the coverage percentage up by then being able to test even the vnu server functionality.

This is obviously a massive rewrite.

Please let me know your thoughts and ideas.