If I have a `_config.yml` that contains:

```yaml
members:
- name: Ben
  username: benbalter
- name: Parker
  username: parkr
```

I'd want to deep-merge it with a `_data/members.yml` with the following contents:

```yaml
- name: Ben Balter
- name: Parker Moore
```

To get the output:

```ruby
site.data['members']
# => [
#  {"name" => "Ben Balter", "username" => "benbalter"}
#  {"name" => "Parker Moore", "username" => "parkr"}
#]
```