Add `Union` Field Type

Basically, allow me to define a union of types that a field can be.

Simple example would be `string | string[]`. Config proposal:

```typescript
// ...
author: {
type: `union`,
of: [
{
type: `string`
},
{
type: `list`,
of: {
type: `string`
}
}
]
},
```

This would permit the following frontmatter:

```yaml
author: Foo Bar
# or
author:
- Foo Bar
- Baz Qux
```

If this is already possible with the existing field types, please add examples to the documentation which demonstrate how to do this, as it's not readily apparent how this can be done.