Improve checking for unterminated string in mirb

mirb fails to check unterminated string when two or more string literals are given.
I can't escape from a code block.

```bash
$ mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> """"
* aaaaa
* """"""
* """
* exit
*
# The code block never ends :(
```

## fixed
```bash
$ mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> """"
line 1: syntax error, unexpected tSTRING_BEG, expecting $end
>
```