Allow subdirectories in _data

Fixes #2082 - Support subdirectories in _data.

Subdirectories in the `_data` folder will also be scanned for `.yaml`, `.yml` and `.json` files, recursively. For more details, see the added Cucumber scenario and the examples below:

Using the following files:
```
_data/categories/dairy.yaml     => name: Dairy Products
_data/categories/spices.yaml    => name: Spices
```

Will result in:
```yaml
categories:
  dairy:
    name: Dairy Products
  spices:
    name: Spices
```