07: Use a switch case in main loop.

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
07: Check if function pointer is not null

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
07: Added sub arithmetic command

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>