// Implement substitution_set and substitution_encode functions
// substitution_set provides a substituion string from the web interface.
// You should use this string to set your own variable that can be used in the substitution_encode function to
// indicate what letters are substituded for what. (Hint: think about the scope of this variable to ensure it can be used)
// The substitution_encode function recieves a the string message from the web interface to be encoded and should return the substituted/cyphered output.