Add kwarg to avoid alternate row shading with az.plot_forest

[drbenvincent]

The default when plotting multiple models with plot_forest is to have alternate row shading, like in the left hand plot here

I agree that this is good default behaviour, but in some situations it adds clutter. @ColCarroll mentioned that you can manually remove it with

ret = az.plot_forest(...); 
for p in ret[0].patches:
  p.set_color('white')

but it would be great if there were a kwarg, such as row_shading or alternate_row_shading which is True by default, but turns it off when setting it to False.

In this case, it resulted in what I think is a superior plot.