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"
}
```