Allow cli pass siteConfig to @vuepress/core

[xqin]

Feature request

@vuepress/core can receive siteConfig parameters, but cli does not declare this command line parameter.

@vuepress/core 模块支持接收一个 siteConfig 参数, 以方便在进行某些测试时, 免除非要在本地创建一个配置文件的烦恼, 但 cli 目前并没有声明这个参数, 导致在命令行中使用时, 无法享受这个便利的功能.

i have been test on local, just modify lib/registerCoreCommands.js, add one line code.

我已经在本地进行过测试, 只需要修改 lib/registerCoreCommands.js 这个文件, 在里面增加一个参数声明即可, 下图为运行效果.

According to this example, we can easily pass an object to cli

https://github.com/cacjs/cac#dot-nested-options

relative issue:

有相关性质的issue

#1955

It’s useful when you’re writing tests and don’t want to depend on actual config file.

https://vuepress.vuejs.org/api/node.html#siteconfig

@yyx990803