Update to webpack 4 and integrate automatic vendor code splitting

Created by: gaearon

https://gist.github.com/sokra/1522d586b8e5c0f5072d7565c2bee693 https://github.com/webpack/webpack/pull/6341

Since we're going to keep 2.x in alpha stage for at least a month or two as we wait for Babel 7 to get stable, we might as well jump straight to webpack 4.

@Ayc0 Seeing that you worked on #3145 and feel passionate about solving this, would you like to work on this?