Add priority option for tasks.

In my situation, I have some tasks which are quite long. This is an edge-case, real or not. In this case, a task may occur in the middle of another task on the same row.

I would like to set a priority level for each task, so that we can intelligently decide if the shorter task should be hidden behind the longer task.

Same as my other pull-request; I don't expect this to be accepted as-is. I'd rather not re-write this to fit your standards, so I'd be happy if this serves only as an idea or as a reference.

Thanks for the great module!