Execute native query command
and if necessary modify the inserted node's className
function _execCommand(doc, command, nodeName, className) {
    if (className) {
      var eventListener = dom.observe(doc, "DOMNodeInserted", function(event) {
        var target = event.target,
            displayStyle;
        if (target.nodeType !== wysihtml5.ELEMENT_NODE) {
          return;
        }
        displayStyle = dom.getStyle("display").from(target);
        if (displayStyle.substr(0, 6) !== "inline") {
          // Make sure that only block elements receive the given class
          target.className += " " + className;
        }
      });
    }
    doc.execCommand(command, false, nodeName);
    if (eventListener) {
      eventListener.stop();
    }
  }