Add ECDF plot and ECDF difference plot with simultaneus confidence intervals as graphical goodness-of-fit and convergence diagnostics

[TeemuSailynoja]

As requested by @avehtari and @hyunjimoon, implement the ecdf plots from this article. Code for the article is available here, so mostly cleaning and formatting needed.

[jgabry]

This would be great!

[hyunjimoon]

Wow thanks, Teemu! I have seen different versions of binomial/beta confidence intervals so having this included in bayesplot would be a great help to users being less confused about SBC overall.

[TeemuSailynoja]

Closed by #282