add scope

This is convenient way to avoid having to add for example:

```javascript
var client = multipub();
//...
client.subscribe('company:user:*');
client.publish('company:user:login', { name: 'tobi' });
```
instead we just do:

```javascript
var client = multipub({ scope: 'company' });
//...
client.subscribe('user:*');
client.publish('user:login', { name: 'tobi' });
```

Thanks!