Allow flag.default and arg.default to be an array if repeatable is true

[DannyBen]

As raised in #461.

This PR adds the ability to use an array for the default value, when using repeatable for both arg and flag:

name: cli
help: Sample application
version: 0.1.0

args:
- name: file
  repeatable: true # must be true, otherwise default array is not allowed
  unique: true
  help: Specify file
  allowed:
  - spaced file
  - other-file
  default:  # <== NEW: allow using an array here
  - spaced file
  - other-file

flags:
- long: --path
  short: -p
  repeatable: true # must be true, otherwise default array is not allowed
  unique: true
  arg: location
  allowed:
  - spaced file
  - other-file
  default:  # <== NEW: allow using an array here
  - spaced file
  - other-file