Upgrade to be a Progressive Web App

[addyosmani]

There's a bunch of work we can do here:

Two biggest changes after the perf improvements in #48 are landing Service Worker caching support and then web app manifest support. For SW, I'd suggest https://www.npmjs.com/package/sw-precache-webpack-plugin however if you're partial to offline-plugin that's also an option.

@yyx990803 Do you have Vue Hacker News iconography or graphics that could be used for the favicons that would get hooked up to the web app manifest?