New script for QA validation of the conversion.
Various fixes and improvements.

    * extract-nvt-desc.py (has_tag_title): New function
    (extract_tag): Make value extraction greedy by stoping at a tag title
    instead of empty line.
    (extract_urls): Also extract urls that start with www. Check for url length.
    (insert_script_tags): Don't insert the version test block if one is found
    already.
    (main): Move tags as global variable.