Implement Modified BetaGeo Model

[ricardoV94]

Very similar to #67, except it allows for dropout before the first purchase: https://github.com/CamDavidsonPilon/lifetimes/blob/master/lifetimes/fitters/modified_beta_geo_fitter.py

[ColtAllen]

I built this model for my btyd library way back in 2022. It sacrifices performance (both in speed and posterior estimation) so that non-repeat customers can have an alive probability < 1. I don't consider this trade-off worthwhile given the effort required to implement this model, and won't be working on it. That being said, if anyone else wishes to do so, they're more than welcome.