task delete should also purge

#### To request a feature...

* Clearly describe the feature.
task delete should have an option or a flag to purge at the same time

* Clearly state the use case. We are only interested in use cases, do not waste time with implementation details or suggested syntax.

In 99 of 100 cases I want to purge a task at the same time as I delete it. Just now I don't actually have a single work flow where I need to keep a task around after it has been deleted. There's no need to also make this verbal with yes/no responses.

* Please see our notes on [How to request a feature](https://taskwarrior.org/docs/features/)