Accept --config option to specify other options

It is boring to write 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.~~

You can specify options by configuration YAML, which is choosed by `--config`(`-c`) option.

## Example

If you create following `config.yml`,

```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 -c config.yml
```