Enable inspection/description of calibration models

[topepo]

Folks might want to look at the fitted calibration model (or have to report the details). They technically can do that with some butchered models but not using things like plot() etc.

Also, we should have a method of extracting the calibrator object directly (like extract_calibrator().

[topepo]

This will require a change to probably, which does the butchering.

[simonpcouch]

Oh, this will be a helpful followup for the testing code in tidymodels/workflows#276!