Add interval expression

[wammaster]

The current version doesn't handle well the interval statement. If we try to validate this SQL code:

SELECT * FROM table WHERE date >= TIMESTAMP_SUB(CURRENT_TIMESTAMP(), INTERVAL 7 DAY)

we lose the day parameter of the interval, here is what is output:

SELECT *
FROM table
WHERE date >= TIMESTAMP_SUB(CURRENT_TIMESTAMP(), INTERVAL 7)

With this PR we will stop losing the unit of time.