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
I'm sorry. I'm confused. Isn't that the information that @jtratner suggested removing last week?

Are you thinking of something like this?

```rst
To specify which writer you want to use, you can pass an engine keyword
argument to ``to_excel`` and to ``ExcelWriter``. The built-in engines are:

- `'openpyxl`': Any compatible version of OpenPyxl. Depending on what version
  of OpenPyxl is installed (1.6.1<= and <2.0.0, or 2.0.0<=), pandas automatically
  selects a version-specific engine, `'openpyxl1`' or `'openpyxl2`' respectively.
- `'xlsxwriter'`
- `'xlwt'`
```