added method that lets caller specify relation between spaced views

original method now calls the new one with NSLayoutRelationGreaterThanOrEqual as before

@ndonald2 I needed this functionality to have some labels display correctly when not pinning their widths; otherwise they'd shrink due to the >= relation of the spacing. This way I can set it to =
Hmm, the fact that the method signatures are semantically different (`itemSpacing` vs `minimumItemSpacing`) makes it a bit unclear that one calls through to the other. I'm wondering if it would be better to just eliminate the original signature in favor of the new one? The unfortunate thing would be that anyone using the original one would have to fix it if they update the repo.