Append Mode for ExcelWriter with "xlsxwriter"
**Problem description**

ExcelWriter's append mode doesn't work when setting default xlsxwriter. To use append mode, I have to set the engine openpyxl. I think it is inconvenient and slow. So I suggest ExcelWriter can be set as append mode when default (xlsxwriter).

**Sample Code**

**AS-IS**
```python
writer = pd.ExcelWriter('APT_TRADE.xlsx', engine='openpyxl', mode='a')
apt_dataframe.to_excel(writer, sheet_name='city-' + date.strftime("%Y%m"), index=False)
```

**TO-BE**
```python
writer = pd.ExcelWriter('APT_TRADE.xlsx', mode='a')
apt_dataframe.to_excel(writer, sheet_name='city-' + date.strftime("%Y%m"), index=False)
```
