Add webpack and webpack-dev-server to build and run the examples
Add screenshot generation via puppeteer (timeout if window.dispatchEvent(new CustomEvent('pex-screenshot')) is not called by the example)
Add screenshot-diff via pixelmatch

Typical dev workflow:

Working on/checking the current status of examples: npm start
Updating live examples on gh-pages (probably on every release):
```
# Start server
npm start
```

Generate all screenshots

npm run screenshots

Build/optimise examples via webpack

npm run build

- Compare code updates via image diff:
```bash
# Generate all (about 1 to 2 minutes) 
npm run screenshots
# or one screenshot(s)
npm run screenshots -- example-name

# Run diff between newly generated screenshot and version on master
npm run diff -- example-name

# See generated file in examples/screenshot/example-name-diff.png

PS: diff testable by changing https://raw.githubusercontent.com/pex-gl/pex-renderer/master/examples/screenshots/${currentExample}.png to https://raw.githubusercontent.com/pex-gl/pex-renderer/examples-screenshot/examples/screenshots/${currentExample}.png

pex-gl / pex-renderer

Examples screenshot #191

Generate all screenshots

Build/optimise examples via webpack