Re-add markdown-it-container (fixes vuejs#1387)

Author: tedmiston

packages/@vuepress/markdown/package.json

@@ -24,6 +24,7 @@
     "markdown-it": "^8.4.1",
     "markdown-it-anchor": "^5.0.2",
     "markdown-it-chain": "^1.3.0",
+    "markdown-it-container": "^2.0.0",
     "markdown-it-emoji": "^1.4.0",
     "prismjs": "^1.13.0"
   },