Implement `JOIN`s

[dnicolodi]

trafficstars

JOINs would be very useful for creating more advanced reports.

Example query:

       SELECT account, bal1, bal2 FROM
         (SELECT account, sum(cost(position)) as bal1
          FROM CLOSE ON 2014-01-01 CLEAR)
         JOIN
         (SELECT account, sum(cost(position)) as bal2
          FROM CLOSE ON 2015-01-01 CLEAR)
         ON account;