Share model List Between Functions

`extract_features`, `download_BERT_checkpoint`, and probably some other functions use the model parameter, with a hard-coded list of models. Investigate listing those models in one place and automatically updating the formals of those functions. Ideally it should still list them all in the documentation of that function (and in RStudio autocomplete), so it'll take at least a little bit of fanciness. I'm pretty certain it's possible, though.