This adds a "dry-run" / "pre-check" option for prune-functions,
which delegates constructing the confirmation message (what is
about to be pruned) and validation of the given options to the
prune-functions.
This helps separating concerns, and doesn't enforce knowledge
about what's supported by each content-type onto the system
prune command.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
a888c4091c