config.json should support comments

[mpsenn]

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.

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