Add compatibility mode for load and store instructions fx55 and fx65 to change Index Pointer

[LakshyAAAgrawal]

Some specifications mention that instructions fx55 and fx65 modify the index register to I + x + 1. Add a command line argument to run the machine with this mode.

[LakshyAAAgrawal]

Relevant: https://stackoverflow.com/questions/51179156/increment-i-in-chip-8-opcode-fx65