to_excel should (optionally) freeze panes
`DataFrame.to_excel()` is currently helpful in styling column and row headers distinctly from values. One other simple feature Excel provides to manage headers is "freeze panes". Panes can be frozen such that the column and row headers are fixed. This is achieved in openpyxl with `worksheet.freeze_panes = top_left_value_cell` and in xlwt with `worksheet.freeze_panes(bottommost_header_row, leftmost_header_col)`.

I think making this available, as an option or by default, would give a valuable usability boost to the exported Excel spreadsheets.