Make line-highlight plugin work with line-numbers plugin

Right nwo the line-highlight and line-numbers plugins don't work together.  The line-highlight plugin first adds the line number for the highlighted lines.  The line-highlight plugin add itself to the code element which is problematic when the code element is positioned relatively (with the line-numbers plugin does).

This change checks for the line-numbers class on the pre element and if it find it, is does not add the data-start/data-end attributes (since the line-numbers plugin is already handling that) and it does appends the line-highlight element to the pre element no matter what (which fixes the position issue.