Allow .itamae to specify options

It is boring to write many options many times. Thus I propose another way to specify options.
With this change, you can specify options with `.itamae` in current directory, which is written in yaml.

## Example

If you create `.itamae` below,

```yaml
host: 192.168.10.10
port: 22
user: user
key: /path/to/private_key
```

you can execute `itamae` without many options.

```bash
# same as: itamae recipe.rb -h 192.168.10.10 -p 22 -u user -i /path/to/private_key
$ itamae recipe.rb
```

## Implementation
Because Thor's required option validation runs when `Thor#new` called, overriding `#options` is not enough.
I avoided the validation by overriding default option of `option` class method.
This line should be here because `Config.default_for` is called when `itamae/cli` required.
`itamae/runner` requires `itamae` and then `itamae/cli`. So `itamae/config` should be upper than `itamae/runner`.