title: R1C1 in openpyxl
body: <p>I'm trying to set conditional formatting in <strong>openpyxl</strong> to emulate highlighting duplicate values. With this simple code, I should be able to highlight consecutive duplicates (but not the first value in a duplicate sequence).</p>
  
  <pre><code>from pandas import *
  data = DataFrame({'a':'a a a b b b c b c a f'.split()})
  wb = ExcelWriter('test.xlsx')
  data.to_excel(wb)
  ws = wb.sheets['Sheet1']
  
  from openpyxl.style import Color, Fill
  # Create fill
  redFill = Fill()
  redFill.start_color.index = 'FFEE1111'
  redFill.end_color.index = 'FFEE1111'
  redFill.fill_type = Fill.FILL_SOLID
  
  ws.conditional_formatting.addCellIs("B1:B1048576", 'equal', "=R[1]C", True, wb.book, None, None, redFill)
  wb.save()
  </code></pre>
  
  <p>However, when I open it in Excel I get an error related to conditional formatting, and the data is not highlighted as expected. <strong>Is openpyxl able to handle R1C1 style referencing?</strong></p>