Be able to set a property (like attr.fill) of an attribute based on the data

We would like to set a color of a marker based on the data.
For example:
```
.map({
  markers: {
    attr: {
      fill: function(d) {
        if (d.status === 'online') return 'green';
        if (d.status === 'pending') return 'orange';
        if (d.status === 'offline') return 'red';
        return 'gray';
    }
  }
})