fix: add webpack to peer dependencies

Hopefully it'll help with #141

Author: SimenB

package.json

@@ -67,7 +67,8 @@
     "webpack-cli": "^3.1.0"
   },
   "peerDependencies": {
-    "html-webpack-plugin": "^3.0.4 || ^4.0.0-0"
+    "html-webpack-plugin": "^3.0.4 || ^4.0.0-0",
+    "webpack": "^4.0.0"
   },
   "engines": {
     "node": ">=6"