weekOfYear
WeekOfYear 添加了 `.week()` API 以返回 `number`,指示 `Dayjs` 是一年中的第几周。
var weekOfYear = require('dayjs/plugin/weekOfYear')
dayjs.extend(weekOfYear)
dayjs('2018-06-27').week() // 26
dayjs('2018-06-27').week(5) // set week
var weekOfYear = require('dayjs/plugin/weekOfYear')
dayjs.extend(weekOfYear)
dayjs('2018-06-27').week() // 26
dayjs('2018-06-27').week(5) // set week