feat: options.devServer can be a func

[dsanders11]

Useful for being able to execute some code specific to the dev server, for example, automatically generating a localhost cert for HTTPS:

devServer: () => {
  // If no localhost.crt, generate it here and output to disk

  return {
    https: {
      key: fs.readFileSync('./localhost.key'),
      cert: fs.readFileSync('./localhost.crt')
    }
  }
}

Would probably be most useful if it could return a Promise, but looking at where it needs to be accessed I'm not sure that's possible in a clean way.