Allow a Task to be executed using an ExecutorService supplied at execution time

org.apache.pivot.util.concurrent.Task can be passed a an ExecutorService at construction which will be used when the Task is executed asynchronously.
http://pivot.apache.org/2.0/docs/api/org/apache/pivot/util/concurrent/Task.html#Task(java.util.concurrent.ExecutorService)

It would be useful to be able to override this (or the default ExecutorService if the no-arg constructor was used) when executing a Task asynchronously, especially for Tasks that might be run multiple times and when the ExecutorService to use is not known when the Task is constructed.