Adds eachIndexPlusOne and a test

So - be warned I did NOT run this. Honestly I did not know how to from source. I totally understand then if you just nuke this pull request. It would bug me. ;) That being said, the mod was so incredibly small I figure if I did make one small mistake it should be quick to fix.

I went with eachIndexPlusOne. Honestly that feels a bit wordy, but I wasn't sure of how best to name it and properly convey the meaning to the dev reading the source code. eachIndexOne is shorter, but sounds confusing ("Is it doing it just once?")