Get build and preview working

[buddhamagnet]

Had a great call with some Cloudflare people today (@buro9 among others). I'm tech lead on the Content Platform at The Economist, and was pointed at this repo as an example of how to get GraphQL running with a Cloudflare worker. It didn't work for me out of the box so I've made some changes:

• Build required webpack-cli.
• Was getting unhandled promise rejections running npm run preview so added some catch statements.
• Finally got it running by removing the chromium app options setting to opn (running on OSX).

Hope this helps!