Add option to in2csv to list sheet names

Created by: khughitt

When converting multi-sheet excel spreadsheets to CSV, the --sheet option for in2csv is needed.

Currently, I don't believe there is any way to list the sheets in the document without viewing it in something external like Excel or LibreOffice.

Would it be possible to add an option to list all available sheets? It may even be worth making this the default behavior for in2csv when multiple sheets are present and no specific sheet is specified -- this way a user doesn't accidentally grab the first sheet without realizing there are others.