Fix playground script

[donalmurtagh]

If you run the script to build the playground

"build:playground": "rimraf build && cross-env NODE_ENV=production webpack --config webpack.config.prod.js --optimize-minimize && cp playground/index.prod.html build/index.html",

It fails because playground/index.prod.html does not exist. I changed this to playground/index.html and the script completes, but the CSS and JS links in build/index.html are incorrect.

I was able to run the playground by manually fixing these links. It would be very helpful if a script was provided that both builds and runs the playground.