Add weights to stat_ecdf

[n-kall]

Currently, stat_ecdf does not support specifying a weight aesthetic to calculate and plot weighted ECDFs. There are a number of third-party implementations that have been made, e.g. ggecdf, stat_ecdf_weighted, stat_ewcdf. However, as ggplot2 continues to develop and evolve, it would be useful to have this as a fully supported feature to ensure it continues being available.