Add verbose mode to mirb

```bash
$ mirb --verbose
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> 1 + 2 + 3
NODE_SCOPE:
  NODE_BEGIN:
    NODE_CALL:
      NODE_CALL:
        NODE_INT 1 base 10
        method='+' (102)
        args:
          NODE_INT 2 base 10
      method='+' (102)
      args:
        NODE_INT 3 base 10
 => 6
>
```