Lemon Squeezy support

[deanpcmad]

This PR adds support for Lemon Squeezy, a Merchant of Record payment provider. Their site is here > https://www.lemonsqueezy.com/

It uses a Ruby library I built, lemonsqueezy.

This is a rewrite of #898 from what I've learnt with integrating Paddle Billing. The Lemon Squeezy API is quite similar to Paddle's Billing API. I have noticed a few things though:

• Trial Subscriptions have the status of on_trial. Not sure if you want to set it as that or replace the status with trialing when saving?
• They don't support cancelling subscriptions immediately.
• Card expiry dates aren't listed in the API, just the brand and last 4
• Charges cannot be created from the API

This is a WIP.