Add hostname support

```
cmd := goclone.Command(...)
cmd.Hostname = "foobar.com"
```
Ahh I just meant that this if clause should have:
} else {
    cmd.hostname = m.pushString("")
}

That way it is either set to c.Hostname (which could be a value, or an empty string) or it will be an empty string.. it won't ever be allowed to be unset.