Fix for demo build - i was unable to run locally

Demo loaded html page with no javascript, it was broken for a few different reasons.
I made 3 changes to fix this.

1. cross-env added so Windows users can run also (cross-env link)
2. small change to webpack config to build html in /build and add script automatically, so script can be removed
3. using ./node-modules/webpack/.bin/webpack is OS incompatible so i added a webpack script:

webpack serve --config ./examples/webpack.config.js --mode development
this ensures webpack runs correctly for whatever environment its being run in as opposed to ./node-modules/webpack/.bin/webpack which won't.

Now npm run demo or npm run start works and displays/runs the demo, and doesn't show a html page with no javascript anymore

All tests pass.

Fixes #293