Offer waiting for a specific download task

It would be great if the end-developer could just wait for a certain task, perhaps by calling something in the likes of `await FlutterDownloader.waitForTask(taskId);`

It wouldn't be too hard to implement – especially after #252 is implemented, there's already a stream of updates coming from the background isolate, so the function could simply create a `Completer` and complete that when the right update event comes through.