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
Good catch. The logic in the v1 engine overlays the style information from the first cell in a merged set with the existing style in worksheet cells, and that test is effectively checking to see if the cell being written to the worksheet defines style information. In the v2 writer, we use `style_kwargs` instead of `style` for the in-loop style data, to make it clear that it isn't yet an OpenPyxl `Style` object. It looks like this is a case where I missed converting `style` to `style_kwargs`.