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.

 
@tsahara Seems like misunderstanding.
I only want to cancel during input line(miss-spelling or any other).
Like irb, simply delete whole line and re-typing from beginning.
```
[koji@macbookpro:~]$ irb
irb(main):001:0> foobarbaz = 33
=> 33
irb(main):002:0> (foobbabaz + 34)*^C
irb(main):002:0> (foobarbaz + 34)*2
=> 134
irb(main):003:0> 
```
```
[koji@macbookpro:~]$ mirb
mirb - Embeddable Interactive Ruby Shell

> foobarbaz = 33
 => 33
> (foobabaz + 34)*
[koji@macbookpro:~]$ 
```
I don't concern about  canceling code execution(currently). 
So this is not related to VM. Related  only to MIRB_READLINE().