Mixin for: adding property to element but zeroing that property on last child.

This is often done (for example, to add space between inline-block or floated elements but to prevent the last from having orphaned space)

``` sass
element
  margin-right: 3px
  &:last-child
    margin-right: 0
```

Looking to replace that with something like:

``` sass
element
  @include all-but-last-child(margin-right, 3px)
```
