Support multiple inputs

This is a first stab at adding support for multiple fields. I try and keep it relatively generalizable, but this leads to some confusing logic (I think).

This PR adds support for multiple inputs for the `name` and `expiry` rendered fields. For name, you can now pass in `firstNameInput` and `lastNameInput` options. For `expiry`, you can now pass in `expiryMonthInput` and `expiryYearInput` options.

Addresses #53, and concerns from @chintanparikh

/cc @joshlevinson @luanmuniz @redcom87