Export momentProperties for plugins

Author: ichernev

moment.js

@@ -1704,6 +1704,10 @@
     // default format
     moment.defaultFormat = isoFormat;
 
+    // Plugins that add properties should also add the key here (null value),
+    // so we can properly clone ourselves.
+    moment.momentProperties = momentProperties;
+
     // This function will be called whenever a moment is mutated.
     // It is intended to keep the offset in sync with the timezone.
     moment.updateOffset = function () {};