Add a `shouldReturn` or `didReturn` hook

[irace]

When part of a larger form, it’s useful to be able to to move directly to the next field once the credit card field is done being populated.

[jflinter]

@irace - agreed, but there are a couple of problems with this.

• The numeric keyboard doesn't have a return key. Inexplicably.
• You're not always certain a user is done typing. In the annoying case of 3-digit amex CVCs, the user may or may not have a 4th digit to type after typing 3 digits.

I've considered adding a method to STPPaymentCardTextField like automaticallyShowInputAccessoryWithTarget:action: that'll wire up a UIToolbar with a done item as the textField's inputAccessoryView, and auto-enable the done item depending on its internal state. Thoughts?

[irace]

Not a bad idea, though also probably should be a pretty low priority.

[jflinter]

Oh, don't worry, it is.