Support priors other than the Minnesota family

Finally we got a request on whether shrinkage priors (Normal-Gamma, Dirichlet-Laplace, etc.) are supported by `bvar()`. I think this would be a cool extension for an expanded package, and should be doable w/out messing up the API too much. However, there's relatively few requests for this feature and implementing it would be a bit involved. Let's see whether there's demand in the future ...