Add support for repeatable array options

[Drowze]

Support repeatable :array options (in addition to comma-separated)

E.g.:

# Given that `my_app` has `api` command taking a `--header` array option
# $ my_app api /some/path \
#     --header 'content-type: application-json' \
#     --header 'accept: application/json'

# Before:
options[:header] # => ["accept: application/json"]

# After:
options[:header] # => ["content-type: application-json", "accept: application/json"]

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Related:

• Add type casting via :cast option #157 (comment)
• More powerful options #129 (comment)