Add custom property to customize placeholder text color

[lpellegr]

While investigating the use of this library, I noticed there is no custom property available to customize the color of the text used as a placeholder in Stripe Elements input texts. However, the Stripe examples show that styling for this use case is supported:

https://github.com/stripe/elements-examples/blob/master/js/example2.js#L43

It looks like a common use case when integrating elements in an existing UI.

[bennypowers]

Should also add one for padding

[bennypowers]

I see we're missing also

• fontStyle
• fontWeight
• text_align
• padding