Add possibility to control extrapolation in ppc_km_overlay() and ppc_km_overlay_grouped()#353

jgabry merged 10 commits intostan-dev:masterstan-dev/bayesplot:masterfrom

Sakuski:ppc_km_overlay-extrapolation_factorSakuski/bayesplot:ppc_km_overlay-extrapolation_factorCopy head branch name to clipboard