privateKey read support.

Hi @z0mt3c

support ssh connect privateKey.

example config
```
beam: {
            stage: {
                servers: [
                    {
                        host: 'xxx.xxx.xxx.xxx',
                        enterCredentials: false,
                        username: 'xxxx',
                        privateKey: './id_rsa',
                        passphrase: 'xxxxxx'
                    }
                ],
                nodeUser: 'root',
                nodeEnv: 'production',
                targetPath: '/home/xxxx/public_html',
                releaseArchive: '../out/backup.tgz'
            }
        }
```

please merge