API Additions
* Add child-management methods and properties to ChildGenerator and
HTMLChildGenerator:
* remove()
* remove_raw() (HTMLChildGenerator only)
* children
* Add new base class NonVoidElement, derive Element from this class.
This base class can be used for elements with content that do not
support the usual container interface.
* Add document-level elements Document, HTMLRoot (<html>), Head (<head>),
Body (<body>), Title (<title>), Meta (<meta>), Script (<script>),
HeadLink (<link>), and Main (<main>).
* Add structural elements Section (<section>), Navigation (<nav>),
Aside (<aside>), Header (<header>), Footer (<footer>), and Heading
(<h1> to <h6>).
* Add list elements OrderedList (<ol>), UnorderedList (<ul>), and
ListItem (<li>).
* Add has_css_class() method to elements.
Improvements
* Element attributes are now always rendered in alphabetical order. This
makes testing elements easier.