Support for bar plots with "positive" and "negative" colors

[lukelbd]

These types of plots are very popular -- where bars are one color for positive data, and another color for negative data. Example:

Something similar is already implemented for fill_between and fill_betweenx (a.k.a. area and areax) via fill_between_wrapperin proplot/wrappers.py.

This could be implemented with a simple override of bar_wrapper.