docs: function keys(path, opts) {
    let env = toUnderscore(path);
  
    if (!opts.caseSensitive) {
      env = env.toUpperCase();
      return Object.keys(process.env).filter(key =>
        key.toUpperCase().startsWith(env)
      );
    }
    return Object.keys(process.env).filter(key => key.startsWith(env));
  }