def optional_string(name, default)
      optional_custom(name) do |env|
        if value = env[name.to_s.upcase]
          {name => value}
        else
          {name => default}
        end
      end
    end
def optional_string(name, default)
      optional_custom(name) do |env|
        if value = env[name.to_s.upcase]
          {name => value}
        else
          {name => default}
        end
      end
    end
Define a required string config var with a default value.