fix: build cannot exit (close: #580)

Author: ulivz

Diff for: bin/vuepress.js

@@ -101,7 +101,9 @@ if (!process.argv.slice(2).length) {
 
 function wrapCommand (fn) {
   return (...args) => {
-    return fn(...args).catch(err => {
+    return fn(...args).then(() => {
+      process.exit(0)
+    }).catch(err => {
       console.error(chalk.red(err.stack))
     })
   }