mirb: Don't exit on Ctrl-C

mirb exits when I press Ctrl-C(SIGINT) during input.
I think it should go back prompt again, like irb.

 
@cremno Are you considering problem for mruby-thread or another threading mrbgems? Then we should implement signal handler using SetConsoleCtrlHandler. If it's not, we can use signal(SIGINT) on windows eventhough.