@remear 
Yeah, I am thinking about multiple period interval. As they are needed, I will add them into this development cycle. 

There might could be a interval for subscription method like this

```python
# charge every 3 months
billy.subscribe(
    customer_guid='XXX',
    plan_guid='XXX (monthly)',
    interval=3,
)
```

@victorlin TODO: add scenario for these cases to #25