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
It looks like there are some changes even between minor versions of v1 (e.g. the border interface) that would make such an exercise quite tedious, and perhaps even futile. Let's leave it be for now with a doc warning. If it wants more development later, we might think about moving in the direction of accepting native style objects rather than building in more and more abstraction, validation, and mapping; make the compatibility layer thinner, not thicker.

In which doc string shall I place the "experimental" flag? `_Openpyxl2Writer`? doc/source/v0.15.0.txt? Other? All of the above?