feat(core): add --open argument to open browser automatically (close: vuejs#1100)(vuejs#1109)

Author: Dmitriy Sharshakov aka. sh7dm

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

@@ -121,6 +121,7 @@ module.exports = async function dev (sourceDir, cliOptions = {}) {
  },
  logLevel: 'error',
  port,
+ open: cliOptions.open,
  add: app => {
  // apply plugin options to extend dev server.
  ctx.pluginAPI.options.enhanceDevServer.syncApply(app)

packages/vuepress/lib/registerCoreCommands.js

@@ -22,6 +22,7 @@ module.exports = function (cli, options) {
  .option('--no-cache', 'clean the cache before build')
  .option('--debug', 'start development server in debug mode')
  .option('--silent', 'start development server in silent mode')
+ .option('--open', 'open browser when ready')
  .action((sourceDir = '.', commandOptions) => {
  const { debug, silent } = commandOptions