feat: added format support to the changelog mode

[alexeytokar]

No description provided.

[jbergstroem]

I think default should be the same as cli (text)

[jbergstroem]

I would validate format here. I was actually about to open a PR that achieves the same where I did something like:

validate_format() {
  local -a output_formats=(githubactions html json junit markup singleline text yaml)
  for f in "${output_formats[@]}"; do
    [[ "${f}" == "${1}" ]] && return 0
  done
  return 1
}

validate_format $format || { echo "Invalid format: ${format}" >&2; exit 1; }

[jbergstroem]

Left some comments; I didn't check open PRs before I attempted to achieve the same. My commit is here: jbergstroem@22a8d89