Proposal: introduces PostHTML into Hexo

Since Hexo is a framework outputting HTML files. So we should introduce an HTML transformer into Hexo's core. PostHTML is a tool to transform HTML/XML. With PostHTML we can:

• Migrate away from RegExp and manipulate PostHTML AST instead.
  • meta_generator: https://github.com/hexojs/hexo/blob/master/lib/plugins/filter/after_render/meta_generator.js
  • external_link: https://github.com/hexojs/hexo/blob/4.2.1/lib/plugins/filter/after_render/external_link.js
  • Injector plugin: https://github.com/hexojs/hexo/blob/master/lib/extend/injector.js
• Enables possibility for more features that require manipulation of HTML (E.g. lazyload images. We can set the data-src attribute directly in the Hexo, and inject the lazyload.js directly to the output, without being limited by the themes)
• Integrate htmlnano directly into Hexo.