Add toggle to simplify show/hide

Perhaps there's an easier way to do this. I'm trying to show and hide a subview (which holds a collection), based on a click event.
If there's not a better way, this seems to work well.

Here's my use case:
view:
```
    events: {
        'click [data-hook~=toggle_activities]': 'toggleActivities'
    },
    toggleActivities: function () {
        var activitiesEl = this.queryByHook('activities');
        dom.toggle(activitiesEl);
    },
    subviews: {
        activities: {
            container: '[data-hook=activities]',
            waitFor: 'model.activities',
            prepareView: function(el) {
                return new CollectionRenderer({
                    el: el,
                    collection: this.model.activities,
                    view: ActivityView
                });
            }
        }
    },
```
template:
```
    <button data-hook="toggle_activities" class="btn">Activities</button>
    <div data-hook="activities"></div>
```