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).