[import_export plugin] Escape ']]>' (closing CDATA tag) when exporting feeds

When an article contains `]]>` (i.e., a CDATA section), the XML produced by the exporter is invalid, which causes importing to fail.

This fix "escapes" `]]>` by splitting it into `]]` and `>` (e.g., `]]]]><![CDATA[>`). (See http://en.wikipedia.org/wiki/CDATA#Nesting)