Add initial webpack config

[kristoforsalmin]

So here's the initial webpack config. It's still missing a lot of things, but for now following needs from #54 were addressed:

• gulp replaced with webpack
• Multiple configs via webpack-merge
• Multiple Browserslist environments (modern and legacy browsers)
• assets-manifest.json is being generated
• .env is loaded via dotenv (there's also cross-env for Browserslist)
• core-js updated
• node-sass replaced with sass

@mi2oon I'm keeping to our present structure with Yeoman for the time being.

That's it so far 🙂 Let me know what you think

[kristoforsalmin]

@mi2oon @sme12 I'm going to merge this one, so it isn't up in the air for too long. Let's continue our conversation about the code splitting in #54 😉