code: 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));
  }
docs: Return a list of environment variables that matches the path provided.
  @private
  @param  {string} path A string separated by dots.
  @param  {Object} [opts] Additional options.
  @return {string[]} An array of environment variables.