Add a butcher argument to the model-based calibration methods

[topepo]

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

Originally posted by @topepo in #65

[simonpcouch]

By "argument" do you mean we should add an argument to axe_*.workflow methods to toggle whether to additionally butcher the calibration objects or just that butchering a workflow ought to butcher the calibration objects generally?

[topepo]

The first option: When we butcher the workflow, we should be able to exempt the fitted calibration model (from lm() or gam()) from being trimmed.