Are recursive types supported?

Is it possible to define a type e.g. "segment" that is either a string, a number, or an array of segments (recursive definition)? If not, please consider it a feature request.

Something like:

```js
const segment = ducktype.indirect();
segment.that = ducktype(String, Number, [segment]);
```

or even:

```js
const segment = ducktype.recursive(self => ducktype(String, Number, [self]));
```
Basically I need to validate that all "leaves" of a non-flat array are of a particular union type. I cannot flatten the array before validating because it is nested in a bigger data structure that I want to validate as a whole (as opposed to picking these arrays out of it for flattening and validation), and the nesting structure within the array is meaningful.