Scally Core -> Base

This is a funny one, it went in yesterday as I saw it in [Inuit](https://github.com/inuitcss/base.images/blob/master/_base.images.scss#L26) and thought it's a good addition as I've been caught out by that many times before. Yes maybe it can live in a separate `vendor` partial? Will add it to the `TODO.md` for discussion.