Custom grid margins (feature request)

Hi,

would it be possible to set the margin as an array to allow setting different margins in one go?

So to use somethink like this for a right-margin only:

```stylus
li
  cell(1, 4, g: (0 2rem 0 0))
```

Instead of this current workaround I'm using:

```stylus
li
  cell(1, 4, g: 2rem)
  margin 0 2rem 0 0
```