Feature : Treat multiple downlaods as a single unit of work

We have a use case where we'd like to treat multiple downloads as a single unit of work and report back progress based on all the downloads in that unit, rather than individually.

Perhaps adding an ability to `enqueue` with a `group id` and query, retrieve using this `group id`, as well as report progress by the group id would be a useful feature for others as well.

So, for example, if have downloads A,B and C. Then I'd like to be able to `enqueue` them all and report progress back as each one completes, so my progress will look like this:

```
A finishes, B and C still running -> Status(running, 33 percent)
A, B finished, C still running -> Status(running, 66, percent)
A,B,C all complete - Status(complete, 100 percent)
```