completion: add configlet completion subcommand

[ee7]

With this commit, running e.g.

configlet completion --shell fish

will output the fish completion script to stdout. This allows us to
distribute completion scripts to the user without adding them to the
release assets, which avoids cluttering every directory in which the
user runs fetch-configlet.

For example, the user can enable completions for fish by running:

configlet completion -s fish > ~/.config/fish/completions/configlet.fish

Closes: #630

---

$ ./configlet completion --shell fish
# global options
complete -c configlet -s h -l help -f -d "Show help"
complete -c configlet      -l version -f -d "Show version info"
complete -c configlet -s t -l track-dir -d "Select a track directory"
complete -c configlet -s v -l verbosity -x -a "quiet normal detailed" -d "Verbose level"

# subcommands with no options
complete -c configlet -n "__fish_use_subcommand" -a lint -f -d "Check the track configuration for correctness"
complete -c configlet -n "__fish_use_subcommand" -a generate -f -d "Generate concept exercise introductions"

# info subcommand
complete -c configlet -n "__fish_use_subcommand" -a info -f -d "Track info"
complete -c configlet -n "__fish_seen_subcommand_from info" -s o -l offline -f -d "Do not update prob-specs cache"

# uuid subcommand
complete -c configlet -n "__fish_use_subcommand" -a uuid -f -d "Output a UUID"
complete -c configlet -n "__fish_seen_subcommand_from uuid" -s n -l num -f -d "How many UUIDs"

# fmt subcommand
complete -c configlet -n "__fish_use_subcommand" -a fmt -f -d "Format the exercise '.meta/config.json' files"
complete -c configlet -n "__fish_seen_subcommand_from fmt" -s u -l update -d "Write changes"
complete -c configlet -n "__fish_seen_subcommand_from fmt" -s y -l yes -d "Auto-confirm update"
complete -c configlet -n "__fish_seen_subcommand_from fmt" -s e -l exercise -d "exercise slug" \
  -x -a "(find ./exercises/{concept,practice} -maxdepth 1 -mindepth 1 -type d -printf '%P\n' | sort)"

# sync subcommand
complete -c configlet -n "__fish_use_subcommand" -a sync -d "Check or update Practice Exercise docs, metadata, and tests"
complete -c configlet -n "__fish_seen_subcommand_from sync" -s o -l offline -f -d "Do not update prob-specs cache"
complete -c configlet -n "__fish_seen_subcommand_from sync" -s u -l update -d "Write changes"
complete -c configlet -n "__fish_seen_subcommand_from sync" -s y -l yes -d "Auto-confirm update"
complete -c configlet -n "__fish_seen_subcommand_from sync"      -l docs -d "Sync docs only"
complete -c configlet -n "__fish_seen_subcommand_from sync"      -l filepaths -f -d 'Populate .meta/config.json "files" entry'
complete -c configlet -n "__fish_seen_subcommand_from sync"      -l metadata -d "Sync metadata only"
complete -c configlet -n "__fish_seen_subcommand_from sync"      -l tests -d "For auto-confirming" -x -a "choose include exclude"
complete -c configlet -n "__fish_seen_subcommand_from sync" -s e -l exercise -d "exercise slug" \
  -x -a "(find ./exercises/practice -maxdepth 1 -mindepth 1 -type d -printf '%P\n' | sort)"

[ErikSchierboom]

LGTM. Seems to work well on my machine