style_indicator_list_sheet: Styles indicator sheet
In ElliottMess/rapporteur: Reports on triple billion data

Used in write_indicator_list_sheet() to style the table.

style_indicator_list_sheet(
  df,
  wb,
  sheet_name,
  start_row,
  end_row,
  start_col,
  end_col
)

`df`	data frame with the indicators to be styled
`wb`	a `openxlsx` workbook to be edited.
`sheet_name`	character name of the sheet to update
`start_row`	integer start row of styling. Passed to `openxlsx::addStyle()`
`end_row`	integer identifying end row.
`start_col`	integer start col of styling. Passed to `openxlsx::addStyle()`
`end_col`	integer identifying end column.