Plot Function for regression residuals

[spsanderson]

I think it might be nice to have a simple plotting function for residuals, right now only regression is implemented for residuals but we can however do observed vs predicted.

Something like this right here should do it but still need to figure out how it will work. Should it work by passing the full model table and using extract_wflw_predictions() inside? should you only pass the output of the extract_wflw_predictions() or should both approaches be supported?