Generalize beta, logbeta, etc. to any number of arguments

Is there interest in generalizing the beta function to [any number of arguments](https://en.wikipedia.org/wiki/Beta_function#Multivariate_beta_function)?  A basic implementation of `logbeta` would be
```
logbeta() = -Inf
logbeta(args...) = sum(loggamma.(args)) - loggamma(sum(args))
```