Feature request

What problem does this feature solve?

this feature can make last update more comfortable if it could support to change date format like 'YYYY-MM-DD hh:mm:ss'

What does the proposed API look like?

API will be based on https://vuepress.vuejs.org/default-theme-config/#last-updated
look like this:

module.exports = {
  themeConfig: {
    lastUpdated: {
        label: 'Last Updated', // string
        format: 'YYYY年MM月DD日 hh:mm:ss' // pattern can refer to moment.js
    }
  }
}

How should this be implemented in your opinion?

emmm... I think may be use moment.js or others...

Are you willing to work on this yourself?

emmm... I'm not good at javascript... but i will do some research... if I resolved by myself. I will push a PR~

sorry for my poor English...
thank you for your reading~