Add unpkg to package

Make the package usable with https://unpkg.com/vue-router
instead of https://unpkg.com/vue-router/dist/vue-router.js

Author: posva

package.json

@@ -75,5 +75,6 @@
     "vue-template-compiler": "^2.1.0",
     "webpack": "^2.2.0",
     "webpack-dev-middleware": "^1.9.0"
-  }
+  },
+  "unpkg": "dist/vue-router.js"
 }