Adding scut-pseudo with shortcuts for display and position

I'm trying an alternative approach of #113 based on how I often use pseudo-elements. What I usually do is use an `@extend` call for a simple combination of `content` and `position` or `display`, making it so these commonly paired properties aren't thrown all over the stylesheet.

This pull request differs from #113 in that it's not meant to create a `:before` or `:after` rule in the stylesheet - these mixins/extends are meant to be used within an existing one. This means that these utilities can much more easily be used via `@extend`. This is not to say that you couldn't provide optional `@content` inside the mixin if you wanted to, but you'd still have to place it inside a pseudo-element rule.

If you think I should rework this to include the `:before` or `:after` rule as well, I could do that. The pro is that for absolutely positioned pseudo-elements I could provide an automatic `position: relative;` for the parent element, but the con is that adding another parameter might mess with the simple names for the extends - there would have to be names like `%scut-pseudo-before-absolute` and `%scut-pseudo-after-absolute` which doesn't seem especially helpful to me.

If you think this is helpful, I'd be happy to write up some documentation on it.