Support groupby:attribute limits

Custom reports are super useful. One thing I am missing is to dynamically filter the tasks in a report. My use case is that I have multiple projects that correspond to different things I do in my life. I would like to fire `task custom-report` and get the first two most urgent tasks from each project so that I can focus on 2*(number of project) tasks for the day. This way I will move all my projects every day.

Right now, to achieve sth like that, I have to manually tag the tasks or do some kind of alchemy so that the urgency of tasks across projects is similar so that with `task next` I will get 2 from each project.

It would be great if there was a hook or some other kind of mechanism which would allow the user to programatically filter the tasks to be reported. This could be a `view` hook that does not affect the task database in any way.