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
Ah. Got it.

Yes, there are situations in which that could happen. For example, v2 recognizes `'size'` and `'sz'` for the font size parameter, but v1 only knows about `'size'`, so `{'font': {'sz': 16}}` would be a valid style specification for v2, not v1. Here's what happens in that case.

For v2:
```python
>>> import pandas as pd
>>> style_dict = {'font': {'sz': 16}}
>>> w = pd.io.excel.ExcelWriter('openpyxl2.xlsx')
>>> style_native = w._convert_to_style_kwargs(style_dict)
>>> style_native
{'font': Font(sz=16.0, u='none')}
```

For v1:
```python
>>> import pandas as pd
>>> style_dict = {'font': {'sz': 16}}
>>> w = pd.io.excel.ExcelWriter('openpyxl1.xlsx')
>>> style_native = w._convert_to_style(style_dict)
>>> style_native
'Calibri':11:False:False:False:False:'none':False:'FF000000':'none':0:'FFFFFFFF':'FF000000':'none':'FF000000':'none':'FF000000':'none':'FF000000':'none':'FF000000':'none':'FF000000':0:'none':'FF000000':'none':'FF000000':'none':'FF000000':'none':'FF000000':'none':'FF000000':'general':'bottom':0:False:False:0:'General':0:'inherit':'inherit'
```

The v1 writer silently ignores the `'sz'` specification. Should this raise instead?