hooks: add preupdate and update hooks.
preupdate run before working dir updated. can prevent update.
update run after working dir updated.
<item>
<title>#tag|escape#</title>
<link>#url#?cs=#node|short#</link>
<description><![CDATA[#tag|strip|escape|addbreaks#]]></description>
<pubDate>#date|rfc822date#</pubDate>
</item>