Support loading extensions

```php
$db = new Clue\React\Sqlite\SqliteClient(':memory:');

$db->on('open', function () use ($db) {
    $db->loadExtension('hash');
});

$db->query('SELECT hash("foo")')->then('var_dump');
```

See https://www.php.net/manual/en/sqlite3.loadextension.php
Refs #47 and #54