Prepare root and element nodes for xast

Ref https://github.com/syntax-tree/xast

- added type: root | element
- renamed elem to name
- replaced "elem" property checks with check for correct type

plugins/removeEmptyAttrs.js

@@ -17,7 +17,7 @@ exports.description = 'removes empty attributes';
  * @author Kir Belevich
  */
 exports.fn = function (item) {
-  if (item.elem) {
+  if (item.type === 'element') {
     item.eachAttr(function (attr) {
       if (
         attr.value === '' &&