showing line number into prompt of mirb

before

```shell
> 
* def foo
* :foo
* end
 => :foo
> 
* foo
 => :foo
> 
```

after

```shell
:001 > 
:002 * def foo
:003 * :foo
:004 * end
 => :foo
:005 > 
:006 * foo
 => :foo
:007 > 
```