Add ES6/ES20xx transpiling for live production frontend builds

[flaki]

ES20xx support is still a hit-or-miss among browsers. This is not a problem for controlled environments, like the backend (with a pre-set version of node.js and ES20xx support), or development-time frontend (where we can eg. say "just use Firefox Nightly").

On live production, most of the drawbacks of transpiling doesn't apply (like source readability and debugging concerns), so this is a pure win for things like #5 (iOS support).

• ES5 transpiling build support (via Babel)
• Additional polyfills (e.g. Fetch API)
• Automatically include polyfills
• Automatic use of dist package in production builds
  (use the --dist commandline parameter or npm run start:prod)