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).
That wouldn't work for [Line 580](https://github.com/jashkenas/underscore/pull/1965/files#diff-304a1162e9f2aef5fb69dd887b585b2bR580), since `_.toArray` would call `_.map`, and `_.map` would call `_.iteratee`.