Custom elements not working as expected
I can see from https://github.com/brainly/html-sketchapp/issues/86 that now custom elements and the shadow dom are now supported. 

Using https://github.com/brainly/html-sketchapp-example and updating the packages to: 

```
"devDependencies": {
    "eslint": "^4.4.1",
    "frontend-tools-configs": "git+https://github.com/brainly/frontend-tools-configs.git"
  },
  "dependencies": {
    "@brainly/html-sketchapp": "^3.3.1",
    "puppeteer": "^1.5.0",
    "webpack": "^4.16.0"
  }
```
Running this script:
```
npm run inject http://webcomponents.github.io/hello-world-element/bower_components/hello-world-element/ .
```

The output in sketch comes up empty. Is there something I am missing? Thanks!