New Tags System

A rewrite of taghandlers so that :

 * tags that used to require forms (looping tags) can now be containing tags.
 * recursive tags work (a tag repeated inside itself)
 * txp:value replaces most tags, except containing, conditional and looping tags
 * <val:article_title /> is a shortcut for <txp:value name="artile_title"/>
 * any value that can be requested by txp:value can also be used as any tag's attribute's values with shortcut <txp:article section="$page_title"/>
 * <txp:link what="article | link | file | comment_author"> replaces permlink; comment_author (with link=1), file_download_url, link, email
 * shortcuts <link:article/> <link:link/> etc.