Update to work with DW master

[rotdrop]

Hi, thanks for the plugin. As it shows the xhtml renderer has changed, in particular many methods now accept an extra boolean argument $returnonly where the expected result is that they don't add to $this->doc but rather return the generated snippet directly to the caller without adding to $this->doc, e.g. `header()':

    /**
     ...
     * @param bool   $returnonly whether to return html or write to doc attribute
     ....
     */
    public function header($text, $level, $pos, $returnonly = false) {
        ...
        if ($returnonly) {
            return $header;
        } else {
            $this->doc .= $header;
        } 
  }