code: public String addClass(String content, String selector, String className) {
  		return addClass(content, selector, Collections.singletonList(className));
  	}
docs: Adds given class to the elements in HTML.
  
  @param content
  HTML content to modify
  @param selector
  CSS selector for elements to add the class to
  @param className
  Name of class to add to the selected elements
  @return HTML content with modified elements. If no elements are found, the original content
  is returned.
  @since 1.0