injectRootOptions (file, options) {
    const _options = (
      this.generator.rootOptions[file] ||
      (this.generator.rootOptions[file] = new Set())
    )
    ;(Array.isArray(options) ? options : [options]).forEach(opt => {
      _options.add(opt)
    })
  }
Add options to the root Vue instance (detected by `new Vue`).