Add external plugin config

[dacsang97]

I added external plugin config for Vuepress base on options variable.
Plugin system can use the local plugin or plugin package in NPM.
.vuepress/config.js

module.exports = {
  ...
  plugins: [
    // Use local plugin. Example: `localPlugin.js` in `.vuepress`
    './localPlugin.js',
    {
      // Use plugin from npm.
      resolve: 'vuepress-plugin-rss',
      // Plugin option
      options: {
        base_url: '/', // required
        site_url: 'https://coralo.io', // required
        copyright: '2018 Coralo' // optional
      }
    }
  ]
}

I just public vuepress-plugin-rss package for generating rss.xml for site using Vuepress site.

[mdaffin]

Similar to and will probably conflict with #196 - we really need to discuss how we want plugin support to work in vuepress. You should add your thoughts to #198.

[ycmjason]

I think regarding plugins, it would be really cool if vuepress could detect plugins itself. Similar to how parcel does it. Maybe as soon as a vuepress-plugin-* is installed in package.json, then load it automatically.

I am not a big fan of configuration hell.

[dacsang97]

Oh, I had seen your work @mdaffin, this is an awesome idea. I think my pull request as the extension of your work, because of I config for both local and remote plugin, so we can public our plugins for the community and reuse them, instead of just using local plugin config.