feat: allow paystack options as argument in initialize function

[kadetXx]

initializePayment function now takes in an optional paystackProps argument as a config param. Props passed in the function's config param will overwrite props passed in the usePaystackPayment Hook.

In hook config options, only the publicKey prop is mandatory since it doesn't change and will be a pain to pass in on every invocation of the initializePayment function. In initializePayment config options, publicKey is omitted.

Fixes issue #78

BREAKING CHANGES: initializePayment function now takes in an object with onClose, onSuccess and config as keys rather than as individual arguments. This ensures all three props can in any order or omitted since they are all optional.