code: function() {
              var self            = this,
                  instruction     = self.parseFilterArgs(arguments),
                  selector        = instruction.command.selector,
                  toggleSelector  = '';
  
              self.isToggling = true;
  
              if (self.toggleArray.indexOf(selector) < 0) {
                  self.toggleArray.push(selector);
              }
  
              toggleSelector = self.getToggleSelector();
  
              return self.multimix({
                  filter: toggleSelector
              }, instruction.animate, instruction.callback);
          }
docs: Adds an additional selector to the currently active filter selector, concatenating
  as per the logic defined in `controls.toggleLogic`.
  
  @example
  
  .toggleOn(selector [, animate] [, callback])
  
  @example <caption>Example: Toggling on a filter selector</caption>
  
  console.log(mixer.getState().activeFilter.selector); // '.category-a'
  
  mixer.toggleOn('.category-b')
  .then(function(state) {
  console.log(state.activeFilter.selector); // '.category-a, .category-b'
  });
  
  @public
  @instance
  @since       3.0.0
  @param       {string}    selector
  Any valid CSS selector (i.e. `'.category-a'`)
  @param       {boolean}   [animate=true]
  An optional boolean dictating whether the operation should animate, or occur syncronously with no animation. `true` by default.
  @param       {function}  [callback=null]
  An optional callback function to be invoked after the operation has completed.
  @return      {Promise.<mixitup.State>}
  A promise resolving with the current state object.