Add option to az.plot_pair to show points from different chains as different colors

[bwengals]

It'd be nice to add an option to plot_pair that would show each chain as a different color. This would help users diagnose whether chains have converged and whether different chains have settled into different modes.

[aloctavodia]

Indeed! This is something that should be easier to do with the new ArviZ (https://github.com/arviz-devs/arviz-plots), once we have plot_pair.

[imperorrp]

@bwengals @aloctavodia I've noted this! Would be good to have in the new plot_pair functionality