Handle has_many :through associations

Extend the deletion tree to find the right scope for has_many :through association.

## Example

### Model
```ruby
class User
has_many :commentators, through: :posts, source: :user
end
```
### Deletion tree

```ruby
module DeleteUser
include Dalek::Exterminate

deletion_tree 'User, {commentators: :delete}
end
```

The issue names no files or tests; start by locating the deletion-tree implementation and its handling of has_many associations. Use the User/commentators example as the acceptance case and confirm that deletion resolves the correct scope for the through association.

Written by the indexing model from the issue text.