Add some ExcelWriter Parameters
The `ExcelWriter` class in line 550 of [pandas/pandas/io/excel.py](https://github.com/pydata/pandas/blob/6c692aee6d411d67997c4fadd023a1d4b7f91c12/pandas/io/excel.py#L550) takes a `datetime_format` parameter to format a datetime column.

Wouldn't it be useful to include a `font_size` parameter as well? Otherwise I don't see how I can set the font size apart from the hard way through `xlsxwriter`. Correct me if I am wrong.
