added activating existing sheet

can activate existing sheet and write more data.    ws = wb.activate_sheet(existing_sheet)

    wb = Workbook()
    ws = wb.new_sheet('test')
    ws[1][1].value = 'first edit'
    ws = wb.new_sheet('test2')
    ws[1][1].value = 'test'
    ws = wb.activate_sheet('test')
    ws[2][1].value = 'second edit'
    wb.save("output.xlsx")