enable the use of folders inside the _layout path

I want to be able to have a more flexible and tidy structure in the _layout folder

something like this

```
├── _layouts
|   ├── posts
|   |   ├── type1.html
|   |   └── type2.html
|   ├── default.html
|   └── post.html
```

And call them in the post front-matter like this

```yaml
---
layout: posts/type1
---
```