Feature request: Environment variables expansion in configuration values

[nvborisenko]

{
  "launch": {
    "name": "My regression suite",
    "description": "Testing from {env:branch} branch"
  }
}

where {env:branch} is special token which is replaced by environment variable with branch name.

Which one is better?

1. {env:branch}
2. {branch}
3. {{branch}}