List sheets and process selected sheets

[harendra-kumar]

Currently we just process all the sheets in a workbook which is not a desirable behavior. Different sheets might have different content which is to be processed differently.

We can provide a sheet selection in the API. The selection could be by index or by name. We will also need an API to list all sheets in the workbook. This is not difficult to implement just a matter of doing it. The sheet listing API already exists in the C code (see https://github.com/harendra-kumar/xls/blob/master/lib/libxls/example/xls2csv.c for usage).

Maybe, we can also provide a simple convenience API for processing the first sheet only.