feat(open_graph): drop 'keywords' option from front-matter

[curbengh]

What does it do?

breaking change, keywords option is dropped from front-matter.

keywords value is added as article:tag meta tag, same as tags value. So, the option is duplicate of tags option in a post's front-matter.

Currently, page.keywords takes precedent over page.tags. To avoid confusion, we should drop page.keywords.

However, keywords can also be specified in site config, which makes it behave like global tags, meaning it will be added to every post's article:tag meta tag, if the post doesn't have a tag. So, config.keywords is still retained.

Previously, there was keywords meta tag in which its value is comma-separated string.

But that tag has been deprecated and replaced by article:tag meta tag in #3805, which is specified one-by-one.

After #3805, as a side effect, open_graph ends up supporting tags and config.keywords in comma-separated string and array types and can lead to confusion (hexojs/site#1288 hexojs/site#1286).

This PR drops support of comma-separated string so that it's consistent with categories.

tags and config.keywords still supports string, but only for single value. It will be converted to a single-element array. e.g. 'foo' -> [ 'foo' ].

How to test

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

Screenshots

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage remained the same at 97.816% when pulling 7f7c571 on curbengh:og-keywords-drop-string into d327789 on hexojs:master.

[SukkaW]

LGTM!

I think it is ready for review now.