Feature: Defined names from workbook returned as Range

[DarkwingDuck48]

Hi!
It would be better to return correspondent Range object, when we use defined_names() method from workbook.
I could share with you my implementation of this, but i'm newbee in Rust and can't write this functionality in optimal way.

My implementation is:

1. Have struct names CellAddress which translate Excel address to tuple with column id and row id
2. Next have struct NamedRange, which have properties name, sheet name and range(tuple of CellAddress objects). This struct have method to construct Range object from corresponds sheet name.

If you interested in my implementation - feel free to visit my repo - smart_csv_converter and look modules celladdress and namedrange.