mysql_aggregate_: allow graphing only max/min value

In some cases it can be useful to know the maximum value of any row of an
aggregate (for example, the maximum number of logins by a single user), without
graphing each row separately (because there are too many or the rows change).
This change allows that by setting the "only" parameter to "max" or "min".
Merge pull request #330 from sipb/mysql_aggregate-max

mysql_aggregate_: allow graphing only max/min value