code: function mergeVisitors(plugins) {
      let result = {};
      plugins.forEach(item => {
          let transform = item;
          let options;
          if (Array.isArray(item)) {
              [transform, options] = item;
          }
  
          typeof transform === 'string' && (transform = require(transform));
  
          Object.keys(transform).forEach(type => {
              let visitor = transform[type];
              let list = result[type];
              list || (list = result[type] = []);
              list.push([
                  visitor, options
              ]);
          });
      });
  
      return result;
  }
docs: Merge the plugin visitors by the visited node type.
  e.g., [ [transform, options], [transform, options] ]
  => {tag: [ [tagVisitor, options], [tagVisitor, options] ]}
  
  @inner
  @param {Array.<Object>} plugins the plugins to merge
  @return {Object}