Added consts for left and right outer/inner joins

Currently you have to pass string and manually specify the join type if it's not included in constants.
AFAIK there are no left / right INNER joins? Right / left joins are specific outer joins.
Therefore, `JOIN_LEFT_INNER` / `JOIN_RIGHT_INNER` aren't meaningful while `JOIN_LEFT_OUTER` and `JOIN_RIGHT_OUTER` MIGHT be acceptable for commonly used, already existing `JOIN_LEFT` / `JOIN_RIGHT`.