Enable deep merging of env settings with default ones

Consider the scenario:

* `default.yml`
```yaml
url:
  base: 'some_url'
  other: 'some_string'
```
* `env.yml`

```yaml
url:
  base: 'another_url'
```

The combining of the above currently produces:
```yaml
url:
  base: 'another_url'
```

It **deletes the extra keys** that might exist on `url`.

If `_.extend` gets replaced with `_.merge`, the result would be the expected:

```yaml
url:
  base: 'another_url'
  other: 'some_string'
```