Adds computed array attribute value support (allows for habtm)

Fixes #31

This PR adds support for computed values within array attribute values. This allows us to define `has_and_belongs_to_many` relationships in seed files like so (assuming Post `has_and_belongs_to_many :tags`):

```yaml
#posts.yml

records:
  - sprig_id: 1
    title: 'My Awesome Post'
    content: 'Lorem ipsum...'
    tag_ids:
      - '<%= sprig_record(Tag, 1).id %>'
      - '<%= sprig_record(Tag, 2).id %>'
```


can use `flat_map` here instead of `.map ... .flatten`