Add support for unique in repeatable args

[DannyBen]

As discussed in #452

This PR allows specifying unique: true for an argument, when it is repeatable: true:

args:
- name: target
  help: Target filename
  repeatable: true
  unique: true   # <-- New option

so that the following command line:

./cli one two one

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