ggdist: Visualizations of distributions and uncertainty

New features and documentation:

• Add curve_interval() for generating curvewise (joint) intervals for curve boxplots (#22)
• Add vignette("lineribbon") describing geom_lineribbon(), stat_lineribbon(), stat_dist_lineribbon(), and curve_interval().

Bug fixes:

• Support dist aesthetics that are factors (#25)
• Fix slab drawing order for overlapping (ggridges-style) slabs (#30)
• Workaround for changes to {distributional} distribution functions until #31 is fixed.