Intercept Custom calls

[raxod502]

Custom is evil and should never be used in Radian. However, a number of other Emacs facilities call into it, such as when setting safe file-local variable values, installing packages with package.el, setting safe themes, and using the actual Customize interface.

We can intercept all such calls by overriding select internal Custom functions, and converting them into something more useful (such as copying a setq or put form to the kill ring).