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
no the subclassing is fine
what I mean is their a situation where a user can try to write a style that openpyxl cannot handle except in version 2 ; so if they have a v 1 write then it should raise (that it's not implemented by openpyxl)