var md = require('markdown-it')({
  html: true,
  linkify: true,
  typography: true
}).use(require('markdown-it-responsive'), options);  // <-- this use(package_name) is required

var option = { responsive: {
    'srcset': {
      'header-*': [ {
        width: 320,
        rename: {
          suffix: '-small'
        }
      }, {
        width: 640,
        rename: {
          suffix: '-medium'
        }
      } ]
    },
    'sizes': {
      'header-*': '(min-width: 36em) 33.3vw, 100vw'
    }
  }
};