Add precise time/date to comments

We've used Timex (#62) to set up timing in the following format (to replicate Github's UI):

However, @nelsonic mentioned (https://github.com/dwyl/github-backup/issues/62#issuecomment-371840575) that having the precise time can be more useful to some people and also Github offers this on hover (on desktop):

This issue is so that precise time can be added.

- [ ] Add precise time on hover of time/date of when a comment has been done

首先定位现有的、基于 Timex 的评论日期格式化代码以及评论显示模板。在保留当前可见格式的同时，添加悬停时显示的精确时间戳，然后验证评论 UI 同时显示相对日期和精确时间。

由索引模型根据 Issue 内容生成。