Support sensible record de-duplication

[oubiwann]

A new command ... maybe dedupe:

Tasks:

• Set up args, command, and dispatch
• Stub out command handler

Features:

• Add command 'list duplicates' #104
• Add algorithm for determining primary record in duplicate #105
• rucksack dedupe --dd-type exact: All records that have the same exact info (name, url, password) but different dates are "joined" with oldest updates becoming the "history" (or being added to the history) of the one with the latest update ts
• rucksack dedupe --dd-type updated: All records that have the name, url, type, etc., but different passwords, are sorted by date and any that are older than the most recent update get added to the history (and "deleted" as independent records)
• rucksack dedupe --dd-type all: Do both, first exact and then updated types
• rucksack dedupe: alias for --dd-type exact