[TW-1916] configurable questions about recurring tasks

_Richard Gay on 2017-06-24T22:51:10Z says:_

Instances of recurring tasks can be annotated and deleted, among other operations. When these operations are performed on an instance of a recurring task, taskwarrior asks (besides the confirmation for the "delete" operation) whether the operation shall be performed on the recurring task itself.

Generally, I like this feature (particularly liked it when I was less familiar with taskwarrior) but would also like to be able to adapt taskwarrior's behavior to my workflow. Concretely:
* I frequently annotate instances of recurring tasks, but I never annotate recurring tasks.
* I frequently delete instances of recurring tasks, but I rarely delete recurring tasks.

This feature could be implemented through a set of taskrc options, particularly:
* `default.annotate.task=("recurring" | "instance" | "ask")`
* `default.delete.task=("recurring" | "instance" | "ask") `

For consistency with the current behavior, the default for both options could be "ask".