MySQL style using clause

MySQL requires parenthesis around the columns in the USING clause, even if there is only one.

join_condition:
    ON conditional_expr
  | USING (column_list)

I wonder if ```(``` and ```)``` should be done at the https://github.com/Wikia/app/blob/dev/lib/composer/wikia/fluent-sql-php/src/Clause/Join.php#L56 level, since it already has something there to insert the commas between the different ```USING()``` calls. I guess this boils down to whether or not this is valid syntax:
```
USING (a,b,c), (d,e,f)
```
which can come up when doing something like this:
```php
$sql->USING('a', 'b', 'c');
if ($someCondition) {
  $sql->USING('d', 'e', 'f');
}
```
If that's valid sql and it works as expected, then it's probably safe to ignore this entire comment :)