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?

Start by reproducing `gren init --token=******` and trace how the init command reads authentication settings. Confirm the token option is handled without requiring `GREN_GITHUB_TOKEN`, and add regression coverage showing that initialization succeeds with the option.

索引モデルが issue の本文から書いたものです。