Ability to mask CVC number

It would be nice if the library could mask the CVC (CVV) number the same way it can mask the card number.

I am assuming this is not already a feature after looking through the code and trying out a few different ways of specifying the CVC number in the `masks` configuration.

Perhaps something like

....

masks: {
cardNumber: '•', // optional - mask card number
cvcNumber: '•'
}

....

Start by locating the existing card-number masking implementation and the `masks` configuration in the library. Trace how CVC inputs are handled, then verify that the requested configuration masks CVC values without changing other fields and add coverage if the existing test structure identifies a relevant location.

Written by the indexing model from the issue text.