Add sort_by 'updated' / 'auto'

[kywk]

'updated': 'updated' in post front-matter first, then 'date' in post.
'auto': 'updated' in post front-matter first, then modified time.

[tommy351]

I think the posts should be ordered by date by default.

[kywk]

Ordered by update provides that you can let some post on top for a period or republish some post to top without change date.
If user use date for their permalink, the changes of date will course origin link broken.

[chenall]

个人认为默认应该按发布的日期（date）排序，而不是按更新日期排序。

[kywk]

Yes, I agree posts should be ordered by date by default.

[aotian16]

貌似这个代码没有合并掉啊

[Xuanwo]

@kywk Can you rebase your code so that I can merge it into master?

[Xuanwo]

Closed for code struct changed.