Add toggle-able fields, Removed focused from onChange

• Now you could use requiresName, requiresCVC, and requiresPostalCode to toggle the displayed fields of CreditCardInput.
• onChange will not receive focused value, because focusing on a field will not trigger onChange event which might cause rage, confusion, and pain when using the focused value
• Now you could use onFocus to properly get the currently focused field (e.g. when you want to display a help text just like this)