Alter mirb exit command into module function

__'exit'__ terminates mirb immediately even though __'exit'__ was overwritten.
```bash
$ mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> exit = 0
 => 0
> exit
```

## fixed
__'exit'__ behaves like __irb__.
```bash
$ mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> exit = 0
 => 0
> exit
 => 0
> quit
```

I also added aliases for __'exit'__:
 - quit
 - mirb_exit
 - mirb_quit