Fix for issue #9671
closes https://github.com/pydata/pandas/issues/9671

`self.style` can be a list of strings or a string. If it is a string, we make a list containing that string, so that we only have to deal with a list of strings. We check every element in that list and raise an error if it has a color symbol.

Thanks for this. A test would be great. You can put it in `pandas/tests/test_graphics.py`.

This will go in .16.1, and we'll start merging PRs for that after .16.0 is released (maybe this weekend).
