Add option to set cycle-rate of CPU-loop

[LakshyAAAgrawal]

Although the specification doesn't specify a rate cycle for the CPU, and only specifies a rate for the DT and ST timers, a lot of Chip-8 programs end up relying on the execution rate of the CPU.

To support both types of programs, a command line argument can be added to the emulator to set the cycle-rate of the emulator(alternatively, the sleep time after every loop iteration).