Added the XPATH command. The first argument is an XPath expression and

Added the XPATH command. The first argument is an XPath expression and
the second, which is optional, is a value to set to the result of the
expression. If no value is specified, the result of the expression
will be sent to the client (if any). Otherwise, the result is set to
the value argument and the document is updated.

Note that this command isn't well tested, but it seems to work. :)