libyang NEW support for user type plugins

Refs #448
diff --git a/src/tree_data.h b/src/tree_data.h
index cf05d81..5480c86 100644
--- a/src/tree_data.h
+++ b/src/tree_data.h
@@ -104,6 +104,7 @@
     uint16_t uint16;             /**< 16-bit signed integer */
     uint32_t uint32;             /**< 32-bit signed integer */
     uint64_t uint64;             /**< 64-bit signed integer */
+    void *ptr;                   /**< arbitrary data stored using a type plugin */
 } lyd_val;
 
 /**