arg: fill_merged_cells. Optional to fill merged cells instead of NAN

[BetterAndBetterII]

issue: #167

I encountered problems when parsing Excel tables. My work is for very large and complex tables with a lot of merged cells with a large span.

If there is no option to parse merged cells, my table is basically unreadable.

I added an arg fill_merged_cells to support parsing merged cells and headers in Excel and filling values ​​into child cells.

[BetterAndBetterII]

btw: how about considering creating a new way to test the various parameters of convert()?