Customized with() methods

[tmtron]

Can we add customized with methods as explained in the immutables docs: "Customize with methods"?

e.g. for a Set we could add with methods like these:

public final ImmutableExampleSetType withIntegers(Integer... elements) {}
public final ImmutableExampleSetType withIntegers(Iterable<? extends Integer> elements) {}

I've already tried this in a new branch of my fork. See this commit