Addition of rug plot to plot_dist

[imperorrp]

Legacy Arviz' plot_dist/plot_kde support rug plot visualization for the density, different from the rug plots for divergences that trace_plot and trace_plot_dist have already. Should this functionality be added to plot_dist with a top level argument to toggle whether to generate it or not?

As an added benefit to other plots that call plot_dist internally, like plot_ppc, the rug plot for the observed_data values could be generated by simply passing rug=True to the plot_dist call made for observed data kde plotting. So the observed_rug plot generation logic could be moved to plot_dist.

[OriolAbril]

Sounds good, it could also be used for the left column of plot_trace_dist, but should be off by default in plot_dist.