Ability to add custom input classes

This makes it easier to add multiple custom classes to text fields and text areas.

Example with using bootstrap forms:

```
add_filter('woocommerce_billing_fields', 'custom_woocommerce_billing_fields');
 
function custom_woocommerce_billing_fields( $fields ) {
  $fields['billing_address_1']['class'] = array( 'form-group' );
  $fields['billing_address_1']['input_class'] = array( 'form-control' );
  return $fields;
}
```