Improve position mixin

It works like if you use margin. It has support for shorthands and has null property to avoid values if you only wanna set some of the values.

Squash at will
The order of these parameters are switched from what it was before (position, coordinates) and I'm thinking this will break anyone's existing code that uses the position mixin.

Additionally, it looks like you're defaulting `$shorthand` to `absolute`. Was there a reason for making this a required parameter with a default value? We already handle the case where someone only specifies coordinates but not the position type on [Line 3](https://github.com/thoughtbot/bourbon/blob/master/app/assets/stylesheets/addons/_position.scss#L3)