Added Support For CXML-DOM (CSS Selectors & XPath)

I created a method to transform trees to the DOM implementation provided by CXML, and made CXML a dependency. CXML-DOM is a fuller implementation of the DOM which enables interaction with libraries like `css-selectors` and `Plexippus XPath`.

Here's an example using CSS selectors:

    (ql:quickload :css-selectors)
    
    (css:query "em"
               (h5:parse-html5 "<p>Hello <em>world</em>.</p> <p>how are <em>You???"
                               :dom :cxml-dom))

I also added h5 as a package nickname, though the value of that addition is debatable.