applyCumulative method on Aggregator

Cumulative sum iterator for Aggregator. I'm not sure if `applyCumulative` is the right name here?
Thanks for the scanLeft implementation, I was being lazy by not using that (for some value of lazy). 

I'd like to push back on having it take `Iterator[A]`, though, because that's inconvenient if you have, say, a `List`. I think the right thing to do is probably to take `TraversableLike` and an implicit `CanBuildFrom`, so that (IIUC), Iterator will return Iterator and List will return List, etc.