Support directory name as a data key

I want to use directory to group some data.

For example:
```
$ tree
.
└─ data
     └── example
           ├── test1.yml
           └── test2.yml
```

At this time, I want to access example data through `data.example`.

Currently, this case could be made by using `example.yml` with keys [test1, test2].
But when each data is too large, it is difficult to treat as one file.