Allow customizing `_.iteratee`.

@jdalton [brought up](https://github.com/jashkenas/underscore/issues/1883#issuecomment-57821014) that unexposing `_.iteratee`'s' `argCount` disallows overriding `_.iteratee` to [customize the way collection functions iterate](https://github.com/jashkenas/underscore/issues/1883#issuecomment-57826279).