code: function(methodName) {
              var self        = this,
                  instance    = null,
                  args        = Array.prototype.slice.call(arguments),
                  tasks       = [],
                  i           = -1;
  
              this.callActions('beforeMixitup');
  
              args.shift();
  
              for (i = 0; instance = self[i]; i++) {
                  tasks.push(instance[methodName].apply(instance, args));
              }
  
              return self.callFilters('promiseMixitup', h.all(tasks, mixitup.libraries), arguments);
          }
docs: Calls a method on all instances in the collection by passing the method
  name as a string followed by any applicable parameters to be curried into
  to the method.
  
  @example
  .mixitup(methodName[,arg1][,arg2..]);
  
  @example
  var collection = new Collection([mixer1, mixer2]);
  
  return collection.mixitup('filter', '.category-a')
  .then(function(states) {
  state.forEach(function(state) {
  console.log(state.activeFilter.selector); // .category-a
  });
  });
  
  @public
  @instance
  @since       3.0.0
  @param       {string}  methodName
  @return      {Promise<Array<mixitup.State>>}