sort by parent projects

Hey there!
I read that some people like to sort by project and then urgency. I struggle with setting that up as sorting by project will also sort by subproject, not only parent project. Me using sub-projects a lot have thus a list basically not sorted after priority at all (but after sub-project name). Using the solidus for the newline after each project yields a report with too many blank lines.

I tried to adjust this with the column variable format `project.parent+/` but that is not a valid sort field. I'd suggest allowing for these formats in the sort order. But I'd be more than happy with a workaround.

* What command(s) did you run?
`task config report.next.sort project.parent+/,urgency-`
* What did you expect to happen?
The next report should now sort by parent project and then by urgency.
* What actually happened?
It doesn't change the configuration because the column formats are not valid fields for the sort order.

The use case would be to fine-tune the sort order even more to one's liking. In my case I'd be able to choose a task to work on by project first, then by urgency or due date even while using subprojects. This is currently not possible in a sensible way since tasks are sorted by parent project and then by subproject alphanumerically vs. by urgency within the parent project.

By the way I got the inspiration to do this from the example workflow page.
> I usually use task to see what's in progress and continue doing that. I have my tasks sorted by projects and priority, so I'll seek the project that needs to advance and check the most important task for that.