Calculate/verify check digit (luhn) of credit cards

``js
// Is the card number valid?
var card = '6123451234567893';

luhn(true, pan); // true


// Calculate check digit

// MII + IIN + "Individual account number"
var incomplete = '612345123456789';

luhn(incomplete); // "3"
``

Implemented according to ISO/IEC 7812-1:2015(E) which goes:

1. Double the value of alternate digits beginning with the first right-hand
digit (low order).

2. Add the individual digits comprising the products obtained in Step 1 to
each of the unaffected digits in the original number.

3. Subtract the total obtained in Step 2 from the next higher number ending in
0 [this is the equivalent of calculating the “tens complement” of the
low-order digit (unit digit) of the total]. If the total obtained in Step 2
is a number ending in zero (30, 40, etc.), the check digit is 0.