Added support for customizing Stripe Checkout parameters

[oscarychen]

This addresses issue #2.

Some of the checkout parameters are specified in DRF_STRIPE settings:

CHECKOUT_SUCCESS_URL_PATH: The checkout session success redirect url path.
CHECKOUT_CANCEL_URL_PATH: The checkout session cancel redirect url path.
PAYMENT_METHOD_TYPES: The default
default payment method types
, defaults to ["card"].
DEFAULT_CHECKOUT_MODE: The default checkout mode, defaults to "subscription".

By default, you can create a checkout session by calling the default REST endpoint my-site.com/stripe/checkout/, this
REST endpoint utilizes drf_stripe.serializers.CheckoutRequestSerializer to validate checkout parameters and create a
Stripe Checkout Session. Only a price_id is needed, quantity defaults to 1.

You can extend this serializer and customize Checkout behavior, such as specifying multiple line_items
, payment_method_types, and checkout_mode.

See README for more info.

[joshuakoh1]

Can you also add coupons?

discounts=[{
  'coupon': '{{COUPON_ID}}',
}]

and promo codes

allow_promotion_codes=True