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/mergePaths.js

@@ -26,7 +26,7 @@ exports.params = {
  * @author Kir Belevich, Lev Solntsev
  */
 exports.fn = function (item, params) {
-  if (!item.isElem() || item.isEmpty()) return;
+  if (item.type !== 'element' || item.isEmpty()) return;
 
   var prevContentItem = null,
     prevContentItemKeys = null;