Make prune in the repository config file not mandatory

[Phothiabea]

According to the README prune must be defined for all repository config but this goes against the idea of both duplicacy and this tool to manage multiple repositories on the same storage location (aka. different snapshot-ids). The documentation of duplicacy recommends only running prune from one repository with the -all option instead of letting each snapshot on the storage being pruned separately. This is even supported by this tool as all defaults to true but running multiple prune --all one after the other is superfluous.

I currently want to back up 3 different repositories to the same storage location and thought I just define the prune -all for one, but I can't do that because it forces me to define a prune for each of my repository config files.