Given a string, replace all X's with 0's and 1's and return all combinations of that string.
node replace.js 10X10X0
returns:
[ '1001000', '1011000', '1001010', '1011010' ]
npm run test
O(2^n) time, where n is the number of X's we are substituting. Best case: Linear time, if there are no wildcards.