Can't alias for global variables.

How to reproduce is like this:

```
$ irb
>> alias $a $__a__
=> nil
>> quit
```

```
$ bin/mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> alias $a $__a__
line 1: syntax error, unexpected tGVAR
> quit
```
