Change Markdown Render Rules to Use Table Header If Present

[michaeljmcd]

Resolves issue #5. The primary change is to add some logic to the render rules for a table element so that the row under thead will be rendered as header, if present, and fall back on the current behavior. Changes to make.ps1 change path separators from backslash to forward slash so that it runs on PowerShell on non-Windows computers.

[cdaven]

Thank you, well done! I will update the test case in test.ps1 and add a note in the changelog.