added support for promotion codes

[gannonh]

In order to support promotion codes, Stripe requires that you set allow_promotion_codes: true when instantiating a checkout session. I made that change and nothing else. Confirmed that the Stripe portal now includes a promotion codes box when checking out.

[RyanTippsTX]

Much needed! i hope this gets merged

[mtliendo]

Instead of this being hardcoded to true, it should be based on a user prop. It looks like it can be added as a prop as a featureArg