Refactor comments using theme_inject

[jiangtj]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

Tested process

• disqus
• disqusjs
• facebook
• livere
• gitalk
• changyan (无法备案)
• valine (不想实名)
• vk (blocked user)

PR Type

What kind of change does this PR introduce?

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build related changes.
• CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the new behavior?

• Use the injection method to implement the comment system.
• Lay the foundation for the multi-comment system.
• Make it easier to plugin

Does this PR introduce a breaking change?

• Yes.
• No.

[jiangtj]

Complexity increasing per file

let theme = hexo.theme.config; is only used in filter, not outside...

[jiangtj]

@stevenjoezhang 蜜汁赞同，都还没测完 😂

It's not finished test yet 😂

[sli1989]

an alter way to rename the post meta of comment for chinese:

for example with valine

1. creat a languages.yml in source\_data. (data files' way)
2. insert following codes: (be carefully about the two-space indent)

zh-CN:
  post:
    comments:
      valine:  评论