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.

 
On Windows this wouldn't even work at ll:
> SIGINT is not supported for any Win32 application. When a CTRL+C interrupt occurs, Win32 operating systems generate a new thread to specifically handle that interrupt. This can cause a single-thread application, such as one in UNIX, to become multithreaded and cause unexpected behavior.

http://msdn.microsoft.com/en-us/library/xdkz3x12.aspx