Allow tracking "nested intervals"

Consider the following use case:

A user is working on multiple projects and is using taskwarrior to track time spent on each project. Let's say that project A uses a ticketing system and the user would like to be able to use taskwarrior to track time spent on each ticket. Since the user is already tracking time on project A, `taskwarrior` can't be used to track the ticket time, because `timew start` will cause the tracking of project A to stop.

To solve that, we could allow "nested intervals" to be recorded. This would turn timewarrior into a "hierarchical" time-tracking system.