#420 Add maxPriority property to CodeNarcAntTask

[Chris Mair]

Add maxPriority property to CodeNarcAntTask – filter out lower priority violations in summary (i.e., don't show counts for priority levels greater than maxPriority).

Note that this does not affect the priorities included in the reports.

This may also provide an opportunity to extend the priorities beyond the current 1..3. In other words, if users configure rules with priority 4, they could include that count in the summary as well.