Extend Subscription length

Hi @caleblai

Thank you for using Paid Memberships Pro.

There may be two possible ways to achieve this:

1. Use custom SQL to update the user’s expiration date.
2. Follow the code from this example and use the pmpro_changeMembershipLevel and alter the user’s current level and then update it via the function mentioned – https://github.com/strangerstudios/paid-memberships-pro/blob/dev/preheaders/checkout.php#L627-L642