feat(open_graph): add article:published_time & article:modified_time

[curbengh]

What does it do?

article:published_time and article:modified_time are listed in the Open Graph spec. The spec doesn't include og:updated_time (removed in more recent spec?), but somehow it's widely adopted (thanks to Yoast?).

Edit: og_updated_time has been deprecated, hence it's not included in the spec. This PR replaces it with article:modified_time.

How to test

git clone -b og-updated https://github.com/curbengh/hexo.git
cd hexo
npm install
npm test

Pull request tasks

• Add test cases for the changes.
• Passed the CI test. (Pending test(open_graph): language is 'en' by default #3660)

[coveralls]

Coverage increased (+0.003%) to 97.27% when pulling a081077 on curbengh:og-updated into 5d94690 on hexojs:master.

[curbengh]

rebased

[curbengh]

Removed article:modified_time as it duplicates og:updated_time.

[SukkaW]

Removed article:modified_time as it duplicates og:updated_time.

og:updated_time seems to be deprecated for a very long time and was replaced by article:modified_time.

Maybe we should use article:modified_time at post and use og:updated_time at other pages, just as hugo does.

[SukkaW]

I will add article:author and article:tag tag for post in a separate PR.

[curbengh]

I've replaced og:updated_time with article:modified_time.

[curbengh]

og:updated_time seems to be deprecated for a very long time and was replaced by article:modified_time.

Maybe we should use article:modified_time at post and use og:updated_time at other pages, just as hugo does.

Since og:updated_time has been deprecated, then we should just remove it.

Should we add article:published_time & article:modified_time to post only? I don't mind whether it's post only or all pages; personally I find it useful to see published and modified time in the html source, even though the page doesn't show it or not an article/post.

[curbengh]

I will add article:author and article:tag tag for post in a separate PR.

Are we going to deprecate keywords? I just looked around, it seems to be deprecated as well.