Refactored building of Nokogiri XML document
Push leading whitespace to outside of elements to convert "* text*" to " *text*" (valid em instead of invalid li).
Refactored adding of children to Kramdown::ElementRt
Added tree traversal and mutation helper methods to Kramdown::ElementRt
Changed how we push out whitespace from spans. Move from process_xml_node to post_process_kramdown_tree. It's way easier to do it in the kramdown tree where many layers of XML nodes have been peeled away already.