Allow exporting variables into the REPL scope

I've added the option to export variables into the REPL scope.

eg.

```php
<?php

require 'vendor/autoload.php';

$b = new \Boris\Boris('test> ', null, array(
  'a' => 42,
));
$b->start();
```


```shell
$ php test.php 
test> $a;
 → int(42)
```