Add parallel option in goji.config.js

[malash]

This PR adds a new option parallel in goji.config.js.

interface GojiConfig {
  ...
  parallel?:
    | {
        minimize?: number;
        loader?: number;
      }
    | number;
}

The parallel.minimize corresponds the parallel option within TerserWebpackPlugin, while the parallel.loader corresponds the workers option within thread-loader.