Allow output escaping to be disabled on an xml.TextNode

XSLT processors like ratago need to be able to disable output escaping on text nodes. libxml2 supports this (in order to support libxslt) but does not offer an API; instead users are required to set the name of the text node to a special value.

This pull request duplicates the logic used by libxslt and exposes it as a function on xml.TextNode.