changed back Format[self] into Formatter
introduced HtmlFormat::PlainNodeFormatter
introduced HtmlFormat::InlineLeafFormatter
removed Formatter[InlineLeaf].visit and Formatter[PlainNode].make_html_element that are unnecessary now
introduced HtmlFormat::HeadingLeafFormatter
removed XhtmlFormat::Formatter[HeadingLeaf].make_html_element that is unnecessary now
introduced HtmlFormat::TableLeafFormatter
introduced HtmlFormat::DescLeafFormatter