Support more styles for xlsxwriter
I was surprised to find that despite the interchangeable representation of Excel styles, xlsxwriter did not have good style support.

I've not added direct tests for this functionality, but test some of it through `test_styler_to_excel`.

 - [x] ~~closes #xxxx~~
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry