Multiple strategies

[portalgun]

Is your feature request related to a problem? Please describe.
I am trying to cleanup a large mail directory in a single command. I have emails that are exactly identical and some with different dates.
I would like to be able to select the older duplicate if dates do not match, otherewise select a random one.

Describe the solution you'd like
Would make sense to use multiple flags like for --hash-header:
--strategy select-oldest --strategy select-one
If a set is going to be skipped, try the next one.

Describe alternatives you've considered
I can write a script to make mdedup run multiple times, but this doesn't seem very efficient --- requires recomputing hashes.