Modify the remit mixin to support lists of sizes.

I've been using Sassifaction for a while, and found myself doing stuff like:

```scss
.foo {
    @include remit(padding, 20);
    @include remit(padding-left, 10);
    @include remit(padding-right, 10);
}
```

I thought it'd be handy if the mixin could handle lists of sizes, so you can do something like:

```scss
.foo {
    @include remit(padding, 20 10);
}
```