CollectionView viewComparator option for custom collection view sorting

See discussion here: https://github.com/marionettejs/backbone.marionette/issues/2056
Yeah, I thought about this, but this is really only short-lived anyways (besides it being totally undocumented).  It only sticks around while the buffer is being built and then is reset when the buffer is rendered, so a user would have to some how jack into it mid-build/pre-render to even access anything useful in there.