[add_data_table] improve rowNames. closes #354

[JanMarvin]

Previously, when using rownames = TRUE when writing data, we would add a cell with 1 in top left box (the name for the rownames). This has been changed:

• when writing data: we do not fill this cell at all.
• when writing a data table: we fill this cell with _rowNames_.

IIRC data tables require unique column names and can not be empty.

• test with Excel
• add unittests
• update NEWS

wb <- wb_workbook()

wb$add_worksheet()$add_data(x = mtcars, rowNames = TRUE)
wb$add_worksheet()$add_data_table(x = mtcars, rowNames = TRUE)

wb$open()