Clone command

Pull for feature request in https://github.com/emre/storm/issues/60: Clone an ssh entry

```bash
$ storm list
listing entries:
    google-01 -> vagrant@google.com:24

    google-02 -> vagrant@google.com:22

$ storm clone google-01 google-03
success  google-03 added to your ssh config. you can connect it by typing "ssh google-03".

$ python storm list
listing entries:
    google-01 -> vagrant@google.com:24

    google-02 -> vagrant@google.com:22

    google-03 -> vagrant@google.com:24
```