Update dependencies

Ref. markdown-it/markdown-it#581.

package.json

@@ -33,27 +33,27 @@
     "@browser-logos/chrome": "^1.0.11",
     "@browser-logos/firefox": "^2.0.5",
     "@browser-logos/safari": "^1.0.4",
-    "autoprefixer": "^9.6.0",
+    "autoprefixer": "^9.6.1",
     "babel-minify": "^0.5.0",
     "cssnano": "^4.1.10",
-    "firebase-tools": "^7.0.0",
+    "firebase-tools": "^7.2.2",
     "glob": "^7.1.4",
     "he": "^1.2.0",
     "html-minifier": "^4.0.0",
-    "husky": "^2.4.1",
+    "husky": "^3.0.3",
     "image-size": "^0.7.4",
     "json-minify": "^1.0.0",
-    "luxon": "^1.16.0",
-    "markdown-it": "^8.4.2",
+    "luxon": "^1.17.2",
+    "markdown-it": "^9.1.0",
     "markdown-it-anchor": "^5.2.4",
-    "markdown-it-attrs": "^2.4.1",
+    "markdown-it-attrs": "^3.0.0",
     "markdown-it-container": "^2.0.0",
-    "markdown-it-footnote": "^3.0.1",
+    "markdown-it-footnote": "^3.0.2",
     "markdownlint-cli": "^0.17.0",
     "minify-xml": "^1.0.3",
-    "post-npm-install": "^1.0.0",
-    "postcss-cli": "^6.1.2",
-    "postcss-custom-properties": "^8.0.10",
+    "post-npm-install": "^2.0.0",
+    "postcss-cli": "^6.1.3",
+    "postcss-custom-properties": "^9.0.2",
     "superstatic": "^6.0.4",
     "workbox-cli": "^4.3.1"
   }