Webpack 4 support

[hgascoigne]

From discussion in #97.

Changes

• Update to webpack 4 and update all webpack loaders
• Transition to @babel scoped packages to support [email protected]
• Replace System.import() with import() so webpack doesn't give a deprecation warning
• Change npm run compile to use stage-2 preset instead of individual transform plugins that are no longer supported

Everything is working for me when I compile, build, or start. Tests don't pass, but they seem to not pass for things that I did not modify. I'm assuming those tests didn't pass before?

Let me know if there is anything you need clarification on or would like changed.

[arielsalminen]

@hgascoigne I imagine this change would drop support for Webpack 3, am I correct?

[hgascoigne]

@viljamis Yes, but only because I made a small change to the webpack config which could be moved into a webpack >= v4 conditional. All of the loader updates were to support v4, but as far as I'm aware are all backwards compatible with v3. Could we make webpack a peer dependency and leave the version decision up to the user?