Support for creating a metered subscription on an existing subscription.

[kyranb]

Cashier currently has the meteredPlan() helper, although with the current documentation it's not clear that one can add a metered plan to an existing subscription by passing in null as the quantity.

subscription('default')->addPlan('plan_price_id', null);

Would it be worth adding an addMeteredPlan() method to make this more readable?

[driesvints]

Good call. I sent this in for the Cashier v13 release which will be released shortly: #1177. Thanks.