himl flattens nested lists

Running `himl` on nested lists results in a flattened list. For example:

```yaml
test:
  - [1, 2]
  - [3, 4]
```

becomes the following after `himl` processes it.

```yaml
test:
- 1
- 2
- 3
- 4
```

I believe that `himl` should not modify the structure of this data.