New attribute modifier - attribute.top:N

This new attribute modifier selects N number of matching tasks, eg. urgency.top:5 .

Example use case: I'd like to be able to cook up a report that included the (example) top 5 highest urgency tasks and (+thistag or +OVERDUE or sched.before:tomorrow) Limit:5 is _not_ the answer here.

As this modifier could be used for any attribute, for natural language queries, I might suggest several synonyms;
- top (highest, latest)
- bottom (lowest, earliest)

and the variable must always be a number;
`urgency.highest:5 or due.earliest:5`
could produce a list of (up to) 10 tasks, fewer if some of the due.latest:5 are also included by the urgency.highest:5