feat($cli): Notify users of a newer release (#2121)

Author: jamesgeorge007

packages/vuepress/cli.js

@@ -1,5 +1,7 @@
 #!/usr/bin/env node
 
+const updateNotifier = require('update-notifier')
+
 const checkEnv = require('./lib/checkEnv')
 const { CLI } = require('./lib/util')
 const registerCoreCommands = require('./lib/registerCoreCommands')
@@ -13,6 +15,7 @@ CLI({
   async beforeParse (cli) {
     const pkg = require('@vuepress/core/package.json')
     checkEnv(pkg)
+    updateNotifier({ pkg }).notify()
     registerCoreCommands(cli, OPTIONS)
     await handleUnknownCommand(cli, OPTIONS)
     cli.version(pkg.version).help()

packages/vuepress/package.json

@@ -36,7 +36,8 @@
     "@vuepress/theme-default": "^1.2.0",
     "cac": "^6.3.9",
     "envinfo": "^7.2.0",
-    "opencollective-postinstall": "^2.0.2"
+    "opencollective-postinstall": "^2.0.2",
+    "update-notifier": "^4.0.0"
   },
   "collective": {
     "type": "opencollective",