gren init with token option

When run `gren init --token=******`, it doesn't work well.
I got following error log.
```
Can't find GREN_GITHUB_TOKEN. Please configure your environment
See https://github.com/github-tools/github-release-notes#setup
```

Can't I use `gren init` with token option?

首先重现 `gren init --token=******`，并跟踪 init 命令如何读取身份验证设置。确认 token 选项的处理不需要 `GREN_GITHUB_TOKEN`，并添加回归覆盖，证明使用该选项可以成功初始化。

由索引模型根据 Issue 内容生成。