Exploratory PR to allow people to adjust the style of the mounted Stripe Elements fields.

[kimcoleman]

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• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
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Changes proposed in this Pull Request:

People with dark themes have no way to target and style the Stripe elements fields that get added via JS. This PR adds a new filter pmpro_stripe_card_element_style so that custom styles can be added.

Here is an example of using this hook to make the placeholder and field values white. You may still need custom CSS to override core PMPro style variations and how background colors and borders are set.

https://gist.github.com/kimcoleman/ab5282830d18252ec39b9dab124d6e7a

[kimcoleman]

Here are the Stripe docs on these styles:

https://docs.stripe.com/js/appendix/style