Added billingCycleAnchor option for subsciption create/update.

Usage example:

$user->subscription("standard")->noProrate()->billingCycleAnchor("now")->swapAndInvoice();

## Stripe Internal Documentation

At creation time, passing a non-now value for billing_cycle_anchor allows you to set up a subscription to have its first "regular" billing date be at a specified point in the future. In particular (and unlike trial_end) billing_cycle_anchor lets you make a prorated charge for a fraction of the month, and then "snap" the billing cycle to the day that you desire.

So, for instance, if on the 20th of January you did a create subscription request with billing_cycle_anchor equal to February 1st, your customer would immediately be charged for about 1/3 of a month, and then subsequently be charged the full amount on the 1st of each month, going forward.

Additionally, when updating an existing subscription, you can pass billing_cycle_anchor=now (you can pass the string "now", just like for trial_end) to force the billing cycle to "reset" and be anchored "now", the way that it would if you were changing intervals.

Concretely, if you had a subscription that billed on the 1st of the month and then on the 18th of February you made an update request with billing_cycle_anchor=now, the subscription would immediately bill in full for the month of February 18th - March 18th, and then bill on the 18th of the month going forward.
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