Prefixing invoices in Membership 2 Pro to create unique Invoice #s

The invoice number generated for PayPal by Membership 2 Pro is a running number based on the IDs in the database.

It would be useful if we were able to prefix the invoice ID with a unique string to create a globally unique Invoice Number.

It would also fix the problem described here:
https://premium.wpmudev.org/forums/topic/membership-2-pro-billing-problem-issue-with-paypal-sandbox#post-914475