Add support for unique in repeatable flag args

[DannyBen]

As discussed in #452

This PR allows specifying unique: true for a flag, when it is repeatable: true and has an arg:

flags:
- long: --path
  arg: location
  help: Set one or more paths
  repeatable: true
  unique: true    # <-- New option

so that the following command line:

./cli --path one --path two --path one

will result in "one two" and not "one two one".