Add order for 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).

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 current behavior?

Order of theme_inject is only determined by the priority of the injection.

What is the new behavior?

Order of theme_inject is determined by the priority of the injection or special configuration in inject api.

Does this PR introduce a breaking change?

• Yes.
• No.

Redesigned the sequential configuration of the multi-comment system.

# Multiple Comment System Support
comments:
  # Available values: tabs | buttons
  style: tabs
  # Choose a comment system to be displayed by default.
  # Available values: changyan | disqus | disqusjs | facebook_comments_plugin | gitalk | livere | valine | vkontakte
  active:
  # Setting `true` means remembering the comment system selected by the visitor.
  storage: true
  # Modify texts or order for any navs, here are some examples.
  nav:
-    #disqus: disqus
-    #facebook_comments_plugin: <i class="fa fa-facebook-official" aria-hidden="true"></i> facebook
-    #gitalk: Load Gitalk
+    #disqus:
+    #  text: Load Disqus
+    #  order: -1
+    #facebook_comments_plugin:
+    #  text: <i class="fa fa-facebook-official" aria-hidden="true"></i> facebook
+    #gitalk:
+    #  order: -2

[ivan-nginx]

Can u provide previous BC configuration? Like in other BC pulls.

[jiangtj]

Can u provide previous BC configuration? Like in other BC pulls.

Okay, done.