Custom aggregations do not appear on final mbtiles

Created an aggregations object like so:

```
aggregations: {
'layer_name': {
count: 'sum',
someField: function myCustomAggregator (memo, feature) {
var newMemo = -1;
// do stuff, works like an Array.reduce() function
return newMemo;
}
}
}
```

The `count` field appears in the output tiles but `someField` does not. In addition, requiring the `geojson-polygon-aggregate/reducers` and using those (`reducers.count()`, `reducers.sum('prop'`) in place of named functions (`'count'`, `'sum'`, etc) also did not produce any new fields.