Implement create customer endpoints

[driesvints]

Create the customer before doing a checkout/subscribe action we no longer have to rely on the billable_id & billable_type keys.

https://docs.lemonsqueezy.com/api/customers#create-a-customer

Revert #52

[heyjorgedev]

Are you working on this @driesvints or can I take a stab at it?

[driesvints]

@heyjorgedev I'd rather tackle this one myself as it'll be quite a crucial part on how the package's gonna evolve. Thanks for offering though!

[driesvints]

@victor-ponamariov please create a separate issue with your question.