Support for php7.1 nullable types

It seems that with the current version (0.8.3) there is no support for [nullable types](http://php.net/manual/fr/migration71.new-features.php#migration71.new-features.nullable-types).

This code:
```php
<?php

function testReturn(): ?string
{
    return 'elePHPant';
}
```
Will throw this error:
> ParseError (syntax error, unexpected '?')