title: Add BOOL property via a category
body: <p>I am using a category <code>NSAttributedString (Additions)</code> and I really need a way to add a property that will be a <code>BOOL</code> (indicating whether the string is an HTML tag or not). I know that categories should not have properties but that's the the way I need to do it. I tired writing my own getters and setters but it didn't work. How can this work?</p>