feat: add configs introduced in Hexo v4

- pretty_urls.trailing_index: https://github.com/hexojs/hexo/pull/3691
- external_link.enable: https://github.com/hexojs/hexo/pull/3675
- meta_generator: https://github.com/hexojs/hexo/pull/3653
- use_date_for_updated: https://github.com/hexojs/hexo/pull/3235

_config.yml

@@ -17,6 +17,8 @@ url: http://yoursite.com
 root: /
 permalink: :year/:month/:day/:title/
 permalink_defaults:
+pretty_urls:
+  trailing_index: true # Set to false to remove trailing index.html from permalinks
 
 # Directory
 source_dir: source
@@ -32,7 +34,10 @@ skip_render:
 new_post_name: :title.md # File name of new posts
 default_layout: post
 titlecase: false # Transform title into titlecase
-external_link: true # Open external links in new tab
+external_link:
+  enable: true # Open external links in new tab
+  field: site # Apply to the whole site
+  exclude:
 filename_case: 0
 render_drafts: false
 post_asset_folder: false
@@ -58,12 +63,18 @@ default_category: uncategorized
 category_map:
 tag_map:
 
+# Metadata elements
+## https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta
+meta_generator: true
+
 # Date / Time format
 ## Hexo uses Moment.js to parse and display date
 ## You can customize the date format as defined in
 ## http://momentjs.com/docs/#/displaying/format/
 date_format: YYYY-MM-DD
 time_format: HH:mm:ss
+## Use post's date for updated date unless set in front-matter
+use_date_for_updated: false
 
 # Pagination
 ## Set per_page to 0 to disable pagination