text: Title: Support for bar plots with "positive" and "negative" colors
  Body: These types of plots are very popular -- where bars are one color for positive data, and another color for negative data. Example:
  
  ![12mo_anom_jul17](https://user-images.githubusercontent.com/19657652/64392145-af3f7c80-d008-11e9-940a-65598d2db804.png)
  
  Something similar is already implemented for `fill_between` and `fill_betweenx` (a.k.a. `area` and `areax`) via `fill_between_wrapper`in `proplot/wrappers.py`.
  
  This could be implemented with a simple override of `bar_wrapper`.