As a shop owner, I can charge quarterly

[chrisjsimpson]

Is your feature request related to a problem? Please describe.

Was checking to see if there is a possibility of changing my current subscribers payments to be made quarterly instead of monthly. Is this possible?

Currently it's only possible to charge one-off, weekly, monthly, and yearly.

Describe the solution you'd like

When creating a subscription, provide the option to charge quarterly.

Describe alternatives you've considered

We could expose the interval_count option to the user for much more flexibly , but will keep simple since the purpose of Subscribie is ease of use- complex yes, but not complicated for the user.

Additional context
Note we don't use the Price object in Stripe (to remain more platform agnostic).
https://stripe.com/docs/products-prices/pricing-models?locale=en-GB#multiple-prices