Add a `defmethod` hook to multimethods to do custom stuff when a new method is added

[camsaul]

One possibility is that multimethods that normally use compound dispatch values can throw an error if you add a dispatch value with the wrong number of args (except for :default).

[camsaul]

#113 would solve the use case here. Once we have that in we might not need this anymore.

[camsaul]

This COULD be done with a ,vanilla?) multimethod that dispatches off of the multimethod. But that doesn't work for programmatic ones. So this should probably be an option passed during creation