By default this function will append any passed argument to the list of modifiers
to be applied, unless the second argument is set to true.
### Example:
Ignore cache query in MySQL
$query->select(['name', 'city'])->from('products')->modifier('SQL_NO_CACHE');
It will produce the SQL: SELECT SQL_NO_CACHE name, city FROM products
Or with multiple modifiers
$query->select(['name', 'city'])->from('products')->modifier(['HIGH_PRIORITY', 'SQL_NO_CACHE']);
It will produce the SQL: SELECT HIGH_PRIORITY SQL_NO_CACHE name, city FROM products