Add missing dependency

Closes GH-2.

Author: wooorm

package.json

@@ -36,6 +36,7 @@
     "parse-entities": "^2.0.0",
     "stringify-entities": "^3.1.0",
     "unist-util-remove-position": "^3.0.0",
+    "unist-util-stringify-position": "^2.0.0",
     "vfile-message": "^2.0.0"
   },
   "devDependencies": {