code: async function mapFiles (customizeResult, targetDir, callback) {
    const files = mergeEngineResults(customizeResult)
    const results = mapValues(files, (file, filename) => {
      // Write each file
      const fullpath = path.join(targetDir, filename)
      return callback(fullpath, file.contents)
    })
    return deep(results)
  }
docs: Map the merged files in the customize-result onto a function a promised object of values.
  
  The type T is the return value of the callback (promised)
  
  @param {object<object<string|Buffer|Readable|undefined>>} customizeResult
  @param {function(fullpath: string, contents: (string|Buffer|Readable|undefined)):Promise<T>} callback functions that is called for each file
  @return {Promise<object<T>>}