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 I mean that windows is possible to handle signal(SIGINT).

```c
#include <stdio.h>
#include <signal.h>

void 
signal_handler(int signo) {
  /* do something */
  puts("stop");
}

int
main(int argc, char* argv[]) {
  signal(SIGINT, signal_handler);  
  while(1) {};
  return 0;
}
```
This code works well on windows.