Extract fenced code from CommonMark documents.

Usage:
  defence [<file>] [options]
  defence -h | --help
  defence -v | --version

Options:
  -n, --no-info         Include fenced code without info strings.
  -i LIST, --info LIST  Include fenced code with given info strings.
  -h, --help            Show this screen.
  -v, --version         Show version.