mapGetters 辅助函数，将一个getter属性另取一个名字

https://github.com/vuejs/vuex/issues/1452

...mapGetters({
  // 把 `this.doneCount` 映射为 `this.$store.getters.doneTodosCount`
  doneCount: 'doneTodosCount'
})
export default {
  computed: mapGetters({
    doneCount: 'doneTodosCount'
  })
}