Implement user-defined weights

[ararslan]

Some stubs are currently in place but we deliberately throw an error when weights are actually specified. The Ferrari 2004 paper makes no mention of weights and I haven't yet bothered to work out the math myself. Off the top of my head, I think what's needed is to account for it in the linear predictor updates, score vector, and expected Fisher information.

Looking at the R documentation for betareg, it appears they only accept case weights. It would likely be easiest to follow suit and use FrequencyWeights from StatsBase (and just wrap any plain input vector with that type). That makes testing against output from R easier.

I almost surely won't block an initial package release on having this feature.