config.json should support comments

As a repository owner of a repository configured to use git2gus, I'd like the `.git2gus/config.json` file to support comments. So that I can know what this oddball config file is doing without documenting it in a separate file.

```json
{
// Reference: https://lwc-gus-bot.herokuapp.com/
// GitHub App: https://github.com/apps/git2gus
"productTag": "XXXXXXXXX",
"defaultBuild": "my default build"
}
```

首先定位读取 `.git2gus/config.json` 的代码，并检查当前 JSON 的解析方式。根据此 issue 中的示例验证所选择的注释支持行为，同时保留 `productTag` 和 `defaultBuild`；完成的标准是 repository 可以在此配置文件中使用注释。

由索引模型根据 Issue 内容生成。