Add ``data`` to extra price fields.

Cart modifiers can add additional text ``data`` beside ``label`` and   ``value`` (this data would be saved in ExtraOrderPriceField and   ExtraOrderItemPriceField models). 

This allows filtering and identifying extra order and extra order item fields by some criteria.

This change is full backward compatible.

I hesitated to serialize extra data as json and tried to keep things as simple as possible.