[completion] _fzf_complete_COMMAND_post for post processing

e.g.

_fzf_complete_foo() {
  _fzf_complete "--multi --reverse --header-lines=3" "$@" < <(
    ls -al
  )
}

_fzf_complete_foo_post() {
  awk '{print $NF}'
}

[ -n "$BASH" ] && complete -F _fzf_complete_foo -o default -o bashdefault foo