fix($cli): re-support option --open (close: #1320) (#1329)

Author: shigma

packages/@vuepress/core/lib/dev.js

@@ -137,6 +137,7 @@ async function prepareServer (sourceDir, cliOptions = {}, context) {
     headers: {
       'access-control-allow-origin': '*'
     },
+    open: cliOptions.open,
     publicPath: ctx.base,
     watchOptions: {
       ignored: [