Add support for flag aliases

[DannyBen]

cc #716, #715

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This PR adds alias support to flags:

flags:
- long: --container
  short: -c
  alias: [-p, --pod]   # <= this
  arg: name
  help: Container name

Aliases are just alternate spellings, like -c. Input values will still only be available in the canonical ${args[--container]}

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