Plugin system

[egoist]

should be flexible enough so we can use it to build another framework, for example a gatsby-like static site generator

Plugin folder structure:

- my-plugin/
  index.js
  enhance-app.js
  enhance-server.js

Use a plugin in config file:

// ream-config.js

module.exports = {
  plugins: [
    ['./plugins/my-plugin', {/* options */}],
    ['ream-plugin-foo', {}],
    '@ream/plugin-bar'
  ]
}