ENH: Conditional  mpl formatting
In line with 
- [ENH: Conditional HTML Formatting](https://github.com/pydata/pandas/pull/10250)
- [Support for Conditional HTML Formatting](http://pandas.pydata.org/pandas-docs/version/0.17.1/whatsnew.html#whatsnew-0171-style)
- [Style](http://pandas-docs.github.io/pandas-docs-travis/style.html)

add the same possibility for mpl tables.

Proof of concept:
[Conditional formatting for 2- or 3-scale coloring of cells of a table](http://stackoverflow.com/questions/17748570/conditional-formatting-for-2-or-3-scale-coloring-of-cells-of-a-table)
