Add INTERVAL items:

> * Add support for ANSI time INTERVAL syntax, INTERVAL '1 2:03:04' DAY TO SECOND
> * Add support for ANSI date INTERVAL syntax, INTERVAL '1-2' YEAR TO MONTH
> * Process mixed ANSI/PG INTERVAL syntax, and round value to requested precision
184a188,189
>   Interpret INTERVAL '1 year' MONTH as CAST (INTERVAL '1 year' AS INTERVAL
>   MONTH), and this should return '12 months'