Sparsify MultiIndex in Styler
#### Merge Duplicate Cells for Easier Reading as a Styler Option
![image](https://user-images.githubusercontent.com/9508385/40321694-bfa0c072-5ce4-11e8-931a-1c608467c206.png)


At work we generate reports in Bi4 and via Python. In the Python reports we write, we use the new Styler object to generate conditionally formatted HTML. 

The generated HTML would be much more readable if there was a way to conditionally merge cells as in the Bi4 report shown above. I really hate Bi4, but the reports do look nicer. 

If this already exists, I apologize, and please kindly point me to the documentation. If not, then thank you for considering this enhancement request.

Best,

\- Joshua

