Support user-defined functions (UDFs)

```php
$db = new Clue\React\Sqlite\SqliteClient(':memory:');

$db->on('open', function () use ($db) {
    $db->createFunction('hash', 'sha1', 1);
});

$db->query('SELECT hash("foo")')->then('var_dump');
```

See https://www.php.net/manual/en/sqlite3.createfunction.php
Refs #47 