Multihop deployment

Support for multhop deployment. I had a situation where I had to go multiple hops and deploy. I submitted a pull request to ssh2 for supporting that. 
https://github.com/mscdex/ssh2/pull/138

For the time being I included the file in the pull request. Let me know what you think.

```
// configuration
plan.briefing({
  debug: false,
  destinations: {
    'production': [
      {
        host: 'www1.pstadler.sh',
        username: 'pstadler',
        agent: process.env.SSH_AUTH_SOCK
        tunnel: {
            host: 'www2.pstadler.sh',
           username: 'vivekg342',
           privateKey: ''<your key>'
           tunnel: {
                   .......
           }
        }
      }  
  ]
  }
});
```