Adds a message to the queue
function addMessage(message) {
  var text = message.content ? message.content.code.trim() : '';
  if (!text) {
    return;
  }

  message.content.code = text;
  _messages.push(message);

  // If there is no message already being processed, go ahead and process this message
  // immediately.
  if (_idle) {
    processNextMessage();
  }
}