Adding in ability to read merge cells from xls and xlsx files.

[tspayne87]

Description

Adding two new methods for both xls and xlsx objects for reading merge cells

• worksheet_merge_cells(name: &str)
• worksheet_merge_cells_at(n: usize)

This was brought about from #305, where I am using the library and need to get merge cells
and I did not see any way todo it in the current codebase. I am a little new to rust so if I am doing
something wrong or if there is a better way of doing something please inform me.

Example

let mut excel: Xlsx<_> = open_workbook(&path).unwrap();
let merge_cells = excel
    .worksheet_merge_cells_at(0)
    .unwrap()
    .unwrap();

[tafia]

There seem to be some issues with the PR. Could you have a look?