docs: function applyClass(token) {
      // init attributes
      token.attrs = token.attrs || [];
  
      // get index of class attribute
      let keys = token.attrs.map(arr => arr[0]);
      let idx = keys.indexOf('class');
  
      if (idx === -1) {
          // Add class attribute if not defined
          token.attrs.push(['class', className]);
      } else {
          // Get the current class list to append.
          // Watch out for duplicates
          let classStr = token.attrs[idx][1] || '';
          let classList = classStr.split(' ');
  
          // Add the class if we don't already have it
          if (classList.indexOf(className) === -1) {
              token.attrs[idx][1] = classStr += ' ' + className;
          }
      }
  }