function env(envName, configSetters) {
  assertConfigSetters(configSetters)

  const currentEnv = process.env.NODE_ENV || 'development'

  if (currentEnv !== envName) {
    return () => config => config
  } else {
    return group(configSetters)
  }
}
Applies an array of webpack blocks only if `process.env.NODE_ENV` matches the
given `envName`. If no `NODE_ENV` is set, it will be treated as 'development'.

@param {string} envName            Environment name like 'development', 'production' or 'testing'.
@param {Function[]} configSetters  Array of functions as returned by webpack blocks.
@return {Function}