In this program you will be using a string to hold "big" integers. The
int type has limits in C++ as you have learned in the past. In this
program you will get practice using arrays and functions to build a
"big" integer adder.

Step 1: Write a function (call by reference, use &) to read in two
  strings which may represent big integers.  The great thing about
  reading the numbers in as strings is that they can be output as a
  string and each individual element can be accessed as an array. One
  catch is that the most significant digit is at index 0. For example,
  if you read in the number 1234 in a string named s then s[0] is
  '1'. This demonstrates the second catch, which is that the "digits"
  are characters rather than numbers. You can convert a character to a
  number using a string of if statements or some clever character
  math. If you use the code: (int)('1' - '0') you will have the
  integer 1 for example.

Step 2: After the last step, you should have two strings that
  represent two integers. These are not ints but rather just an array
  of characters that represent the digits of an integer. Write a
  function (call by value) that adds two of these integers
  together. You will have to add the strings starting at the end of
  the arrays. There are two ways to do this. One is to write a clever
  while loop that starts at the end of both arrays and makes sure to
  never "walk off" the beginning of either string. Perhaps an easier
  way is to reverse the strings before adding them and then simply
  reverse the result.

  As an example:
    1234   is stored as 4 digits in the string '1' '2' '3' '4'
    56     is stored as 2 digits in the string '5' '6'

  To add these together I simply cannot start at index 0. But rather I
  need to keep track of two separate indicies that mark the start of
  each number individually (or do the reverse trick). You can get the
  last index using string.length() - 1

  Remember to consider carry. 6+4 = 10 means that the digit in the
  ones place of the result is '0' but that the tens place is 5+3+1 = 9