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
@neirbowj took a look at this a second time and realized I'd missed this - `style` isn't defined here - what is it supposed to be?