DOC: "justify" parameter in to_html
The Travis docs for <a href="https://pandas-docs.github.io/pandas-docs-travis/generated/pandas.DataFrame.to_html.html">to_html</a> states that the only accepted parameters for `justify` are `left` and `right`.

However, I stumbled upon the fact that you can also pass in `center`, and it will work just fine (i.e. the cell elements and headers are centered), as evidenced by the code:

https://github.com/pandas-dev/pandas/blob/fa557f7391589f351b1260f46b3b3db22492f50b/pandas/io/formats/format.py#L1215-L1217

@TomAugspurger : Should we continue to allow only `left` and `right` or should we expand to accept `center` ?  I'm personally up for the latter but wanted to get consensus first.