Add getWebpackConfig hook and document how to use custom webpack config #252

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Feature request

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What problem does this feature solve?

You can directly modify webpack config in this hook. Easier than current chainWebpack hook.

What does the proposed API look like?

In saber-node.js:

exports.getWebpackConfig = (config, { type }) => {
  config.plugins.push(new MyWebpackPlugin())
  return config
}