Allow for token on Stripe.Customer.create/1 / Stripe.Customer.create/2

[Kartstig]

According to the Stripe Customer Documentation, a token can be passed into this endpoint which will create a new default payment source for the customer:

Currently, this method accepts a Stripe.Source.t(), but that would require an additional HTTP request. Instead, if we allow a binary type for the param token then we can let Stripe handle the downstream Source creation.

I'm currently getting around this by doing the following:

Stripe.Customer.create(%{source: %Stripe.Source{token: "tok_visa"}})

This allows the token to be passed when it encounters cast_id

However, this isn't really clear in the docs. If we don't want to change the type contracts to accept a binary() for source, then maybe we should indicate this solution in the documentation. Also, there are no tests that pass parameters to this function, the only test for create passes an empty map.

[Kartstig]

It seems to be working fine with the specified workaround, however, Dialyzer is yelling at me