ENH: Implement _Openpyxl2Writer for pandas.io.excel

This patch is an implementation of a ``pandas.io.excel.ExcelWriter`` subclass that is compatible with the style interface of ``openpyxl`` 2.0.0 or later.

closes #7177
ideally u could raise or want to avoid confusion - if I can do it in a fairly generic way

v1 should work the way it works now (and anything else is an error)

no biggie if this is not possible - just maybe a doc warning 

and FYI / I would make the v2 support experimental (put in the doc string) - if haven't already - thT way a bit of license to change in the future if needed