Support yaml as node

Node object is useful but json format is not so useful to write.
I'm happy if itamae supports yaml format for node object.

## Example

```bash
$ cat node.yml
user: hoge
$ itamae execute -y node.yml recipe.rb
```

is the same as:

```bash
$ cat node.json
{
  "user": "hoge"
}
$ itamae execute -j node.json recipe.rb
```