add function xmlTextReaderRelaxNGValidateCtxt()

Since there is xmlTextReaderSchemaValidateCtxt() it seems like there
should be an equivalent RelaxNG function. The attached patch adds it.
The code is essentially the same as Schema implementation, but I'm
uncertain as to how to add things to the documentation and test suite:
there seems to be a lot of auto-generation going on.

include/libxml/xmlreader.h

@@ -284,6 +284,11 @@ XMLPUBFUN int XMLCALL
 XMLPUBFUN int XMLCALL
 		    xmlTextReaderRelaxNGValidate(xmlTextReaderPtr reader,
 						 const char *rng);
+XMLPUBFUN int XMLCALL
+		    xmlTextReaderRelaxNGValidateCtxt(xmlTextReaderPtr reader,
+						 xmlRelaxNGValidCtxtPtr ctxt,
+						 int options);
+
 XMLPUBFUN int XMLCALL
 		    xmlTextReaderRelaxNGSetSchema(xmlTextReaderPtr reader,
 						 xmlRelaxNGPtr schema);