feature: 增加 tags 標簽外掛 - label

improvement: 優化 pjax 下 404 頁面

layout/includes/404.pug

@@ -0,0 +1,17 @@
+- var top_img = theme.error_404.background || theme.default_top_img
+- var bg_img = `background-image: url('${url_for(top_img)}')`
+
+#body-wrap.error
+  div(style='display: none')
+    include ./header/index.pug
+
+  #error-wrap
+    .error-content
+      .error-img(style=bg_img)
+      .error-info
+        h1.error_title= '404'
+        .error_subtitle= theme.error_404.subtitle
+        a.button--animated(href=config.root)
+          i.fas.fa-rocket
+          = _p('error404.back_button')
+