introduced @generator=HtmlElement in class HtmlFormat
removed the redefinition of XhtmlElement#create_element
removed the redefinition of XhtmlElement#visited_result
introduced HtmlFormat.setup_new_formatter
added class Xhtml5Format
refactoring of HeadingNodeFormatter#create_self_element
modified HtmlElement#add_end_comment_for_div and and renamed to add_end_comment_for_div_or_section to reflect the modification
added a test for Xhtml5Format