Added optional command line argument to set cycle-rate of CPU-loop

[BhavyaC16]

Provided the cycleRate attribute to the Machine class. An optional argument can be passed to the emulator in microseconds to control the CPU loop rate, which is otherwise set to 10 by default.

Example to set cycle rate to 5000 microseconds:

./bin/c8emu ./roms/games/Tetris\ \[Fran\ Dachille\,\ 1991\].ch8 5000

Closes #2