Enable webpack v2 performance hints

• Add lint rule to disallow require.ensure (https://github.com/facebookincubator/create-react-app/pull/1536)

• Add support for dynamic import() (#1538)

• Document dynamic import

• Tune performance hint thresholds

• https://github.com/webpack/webpack/issues/4292

[sokra]

btw. you can disable require.ensure in webpack 2.

module: {
  rules: [
    { parser: { requireEnsure: false } }
  ]
}

[Timer]

We should probably watch https://github.com/webpack/webpack/issues/4292 (unrelated to this issue, but something to not forget about when cutting 0.10.0).