fix: add webpack peer dependency

Author: kmontag

Diff for: package.json

@@ -40,6 +40,9 @@
     "uglify-loader": "^2.0.0",
     "webpack": "^3.8.1"
   },
+  "peerDependencies": {
+    "webpack": "^2.0.0 || ^3.0.0"
+  },
   "repository": {
     "type": "git",
     "url": "https://github.com/kmontag/protobufjs-loader.git"