Use customStringLabel as a new field name and customString as a value

[dsvetlov]

I tried o do it, but as I understand it is possible only with ruby code. I suggest to imlement it in this filter. Now in the output of cef filter we receive such documents: {"customDeviceString1 => "foo", "customDeviceString1Label => "bar"}. I suggest at lest to add an option wich will produce such documents {"bar" => "foo"} instead of prevous one.