Add `bounds` argument to plots building on `geom_density` and `stats_density`

[TeemuSailynoja]

At the moment, these functions use geom_density, or stat_density

• ppc_dens
• ppc_dens_overlay
• ppc_dens_overlay_grouped
• ppd_dens
• ppd_dens_overlay
• mcmc_dens
• mcmc_dens_overlay
• mcmc_dens_chains
• mcmc_areas
• mcmc_areas_ridges

They all can feasibly be used to plot, for example, non-negative values, and the plots can look quite different if the bounds parameter is passed to stat_density.

[jgabry]

Good idea!